I was looking a the "g-series-5090-desktop_service-manual" and it seems like the motherboard only comes with:

1x m.2

1x 2.5 sata

If the original m.2 is failing ( assume you're not going to use the original card anymore). Here are my suggestions :

1) backup the files and get a new m.2 card, do a clean installation of windows and move the files to the new m.2

OR

2) get a ssd and install with new windwos and install it in the 2.5" slot , so you can keep the old m.2 , 2.5" slot as shown in below

I hope this is the same version. If not, please take a photo inside your case (mainly the motherboard) and see if there's any possible free slot for extra storage.

Hope this help.

Thank you for taking an interest in my dilemma.  Is there anything that can be done to get the Kioxia back to original performance levels?  I feel that an NVMe should continue to perform well after only 18 months of use.

To be fair, read speeds are still pretty good (around 1,260 MB/s), but, as you can tell from the original post, I find the situation around sustained writes very distressing.  I might be able to get around 89.1 MB/s on a good day.

SSD drives performance goes down the more errors it gets until it dies completely.

This is known.  SSD's have a maximum number of writes and then they are dead.

TRIM is to allow the Operating System to communicate to the SSD that a block of data is no longer needed or being used by the file system (garbage collection). If TRIM is not functioning there will be a drop in performance similar to ecc errors but this too will get worse over time as the drive runs out of over provisioned blocks.

If the TBW is up, then it means it’s time for your m.2 to retire or not to be used for main hard drive as it will not perform as expected and increase the risk of disk failure. Yet, to determine what is the actual problem, I recommend to use “crystal disk info” and screenshot the details (mainly the top right section). Then we can determine is it’s TBW is used up. 

----------------------------------------------------------------------------
CrystalDiskInfo 8.12.0 (C) 2008-2021 hiyohiyo
Crystal Dew World: https://crystalmark.info/
----------------------------------------------------------------------------

OS : Windows 10 [10.0 Build 19041] (x64)
Date : 2021/05/01 12:29:57

----------------------------------------------------------------------------
(04) KBG40ZNS1T02 NVMe KIOXIA 1024GB
----------------------------------------------------------------------------
Model : KBG40ZNS1T02 NVMe KIOXIA 1024GB
Firmware : 10410104
Serial Number : X9GPE6NKPTNL
Disk Size : 1024.2 GB
Buffer Size : Unknown
# of Sectors :
Rotation Rate : ---- (SSD)
Interface : NVM Express
Major Version : NVM Express 1.3
Minor Version :
Transfer Mode : ---- | ----
Power On Hours : 11100 hours
Power On Count : 173 count
Host Reads : 7006 GB
Host Writes : 13523 GB
Temperature : 52 C (125 F)
Health Status : Good (97 %)
Features : S.M.A.R.T., TRIM, VolatileWriteCache
APM Level : ----
AAM Level : ----
Drive Letter : C:

-- S.M.A.R.T. --------------------------------------------------------------
ID RawValues(6) Attribute Name
01 000000000000 Critical Warning
02 000000000144 Composite Temperature
03 000000000064 Available Spare
04 000000000032 Available Spare Threshold
05 000000000003 Percentage Used
06 000000E03661 Data Units Read
07 000001B0BC62 Data Units Written
08 000011473770 Host Read Commands
09 000022FBC2B9 Host Write Commands
0A 000000000A66 Controller Busy Time
0B 0000000000AD Power Cycles
0C 000000002B5C Power On Hours
0D 000000000000 Unsafe Shutdowns
0E 000000000000 Media and Data Integrity Errors
0F 000000000000 Number of Error Information Log Entries

-- IDENTIFY_DEVICE ---------------------------------------------------------
0 1 2 3 4 5 6 7 8 9
000: 1E0F 1E0F 3958 5047 3645 4B4E 5450 4C4E 2020 2020
010: 2020 2020 424B 3447 5A30 534E 5431 3230 4E20 4D56
020: 2065 494B 584F 4149 3120 3230 4734 2042 2020 2020
030: 2020 2020 3031 3134 3130 3430 8E03 8CE3 0900 0000
040: 0300 0001 A120 0007 3450 0003 0200 0000 0002 0000
050: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
060: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
070: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
080: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
090: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
100: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
110: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
120: 0000 0000 0000 0000 0000 0000 0000 0000 0017 0703
130: 0E14 04FF 0100 0163 0167 0000 3D00 0000 1700 0000
140: 6000 77A5 00EE 0000 0000 0000 0000 0000 0000 0000
150: 0000 0000 0000 0000 0000 0000 0000 0000 0031 0101
160: 0000 0001 0000 0163 0002 0000 0000 0000 0000 0000
170: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
180: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
190: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
200: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
210: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
220: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
230: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
240: 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000
250: 0000 0000 0000 0000 0000 0000

-- SMART_NVME --------------------------------------------------------------
+0 +1 +2 +3 +4 +5 +6 +7 +8 +9 +A +B +C +D +E +F
000: 00 45 01 64 32 03 00 00 00 00 00 00 00 00 00 00
010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
020: 5A 37 E0 00 00 00 00 00 00 00 00 00 00 00 00 00
030: 8B BD B0 01 00 00 00 00 00 00 00 00 00 00 00 00
040: FA 7D 47 11 00 00 00 00 00 00 00 00 00 00 00 00
050: EB EA FB 22 00 00 00 00 00 00 00 00 00 00 00 00
060: 66 0A 00 00 00 00 00 00 00 00 00 00 00 00 00 00
070: AD 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
080: 5C 2B 00 00 00 00 00 00 00 00 00 00 00 00 00 00
090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0A0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0C0: 00 00 00 00 00 00 00 00 45 01 00 00 00 00 00 00
0D0: 00 00 00 00 00 00 00 00 09 00 00 00 00 00 00 00
0E0: 09 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
110: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
120: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
130: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
140: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
150: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
160: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
170: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
190: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
1A0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
1B0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
1C0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
1D0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
1E0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
1F0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

Yours:

Health Status: 97%

Power On Hours : 11100 hours
Power On Count : 173 count
Host Reads : 7006 GB
Host Writes : 13523 GB

Mine:

Health Status: 88%

Power On Hours : 1056 hours
Power On Count : 83 count
Host Reads : 100864 GB
Host Writes : 95703 GB

It doesn't look like a TBW issue to me as your ssd's health status is 97%.

Whats the actual read/write speed of your ssd currently?

you can use CrystalDiskMark8 or ATTO Disk Benchmark to generate the report.

Please save the report as image and share with us.

If the read/write is also normal, it might be lack of storage.

------------------------------------------------------------------------------
CrystalDiskMark 8.0.1 x64 (C) 2007-2021 hiyohiyo
Crystal Dew World: https://crystalmark.info/
------------------------------------------------------------------------------
* MB/s = 1,000,000 bytes/s [SATA/600 = 600,000,000 bytes/s]
* KB = 1000 bytes, KiB = 1024 bytes

[Read]
SEQ 1MiB (Q= 8, T= 1): 2392.342 MB/s [ 2281.5 IOPS] < 3274.73 us>
SEQ 1MiB (Q= 1, T= 1): 1819.271 MB/s [ 1735.0 IOPS] < 575.85 us>
RND 4KiB (Q= 32, T= 1): 471.758 MB/s [ 115175.3 IOPS] < 268.98 us>
RND 4KiB (Q= 1, T= 1): 52.238 MB/s [ 12753.4 IOPS] < 78.26 us>

[Write]
SEQ 1MiB (Q= 8, T= 1): 469.441 MB/s [ 447.7 IOPS] < 15929.56 us>
SEQ 1MiB (Q= 1, T= 1): 1011.062 MB/s [ 964.2 IOPS] < 1027.09 us>
RND 4KiB (Q= 32, T= 1): 297.520 MB/s [ 72636.7 IOPS] < 421.02 us>
RND 4KiB (Q= 1, T= 1): 75.194 MB/s [ 18357.9 IOPS] < 54.31 us>

Profile: Default
Test: 1 GiB (x5) [C: 71% (666/936GiB)]
Mode: [Admin]
Time: Measure 5 sec / Interval 5 sec
Date: 2021/05/03 18:47:31
OS: Windows 10 [10.0 Build 19041] (x64)

Were you running other programs while you're doing the test?

The data may indicates some background programs were running and continuously using the ssd.

The SEQ 1MiB (Q=8, T=1) and RND 4KiB(Q=32, T=1) seems too slow compared to the reading speed.

Negative, I made sure there was no I/O activity for 60 seconds before starting the test.

try change the RAPID mode in BIOS, if you have it enabled, turn it off, if it's off, enable it.

PS. it would be useful if you can also upload a task manager - processes screen shot, just in case we might miss something with the background softwares.

1. To enter the BIOS Setup Utility, restart your computer and immediately begin pressing F2 at6 the Dell logo.
2. Use your Arrow keys to go to the Performance or Advanced BIOS menu and pressEnter.
3. Use your Arrow keys to go to the Intel Rapid Start Technology menu and press Enter.
4. If available, use your Arrow keys to go to the Rapid Start Timer or Critical Battery options.
5. Use the Plus (+) and Minus (-) keys to change the mode to Enabled.
6. Save your settings and exit the BIOS Setup Utility.
7. Restart your computer.