I should add to this that I have only recently reistalled SupportAssist due to another issue.

Because of issues like what you are encountering, I don't recommend installing Support Assist. You can find that in this forum others have had issues with Support Assist. Instead of using Support Assist, I check Driver & Downloads for my system on the Dell support site for updates and decide whether or not updates need to be installed. Usually, if the system is working fine it is better to leave it alone. 

I have an XPS 13 7390 2-in-1, with May 2020 update (that caused problems).

Short version - I got this, repaired SupportAssist, and the update disappeared, I think without having been installed.

I also got this update flagged yesterday.  The first time, I got the same as you, maybe a second time.  After that, with a restart I think, it got further and seemed to be downloading.  I had to restart as the system went to sleep (the May 2020 update really does not like the system sleeping).  Then a hang like you.  Then it went as far as downloading and installing.  Then another restart and I think another hang like you.  Then it went all the way to a "could not install" or "failed to install" with a note to look at the right-hand column for a reason (there was nothing there).  Then another restart (as I said, Windows 10 2004 really, really does not like this system going to sleep) and I decided to look at the application itself as a last thing before contacting Dell Support.  Typed SupportAssist into the Cortana box; without opening it on the right there's an "App settings" option, I clicked that.  Scrolled down, there's a "repair" option.  I did that and since then the update has not appeared.  Whether this is all as expected or not I have no idea.

I am now going to roll back to the 1909 build.

I've gone back to 1909 now.  I think there's something wrong with SupportAssist in 2004, as a result, running it now and it's taking longer but I think that means it's actually doing the tests.  It seemed to skip the hardware tests in 2004.  It has picked up this Recovery Tools update again.  I'll try installing it and resetting it as before, depending on what happens.

This time the update installed without trouble.

Note regarding my comment where I said it downloaded and installed - it didn't install, it tried to install.  The only time it did actually seem to install was with 1909.