Graphical glitches with Raspberry Pi 400 + LinuxCNC 2.9.4 (arm64)

Have you implemented the zswap fix or tried looking at the memory allocated to the GPU ?

To be quite blunt this is a low priority issue, as the primary function of the image is to run a CNC machine. Which by all accounts it has done quite well, the only show stopper was the zswap issue for one user, but this was resolved.
I may sound like an ass and appear to be unhelpful, but as I said before it's a low priority issue and so far has not, no reports, has affected the operation of a Linuxcnc session.

The zswap fix seems intended to fix a system log error message "Failed to start zramswap.service" but I checked systemctl and have no failed units, so it doesn't seem relevant to this case, is it?

Is there a guideline on how much memory to allocate to the GPU? This is a 4GB Pi 400. One idea I have is to compare the config.txt with that of other images that work well.

The other thought is regarding codecs or configuration that may be missing as suggested by the attached report from the chrome://gpu

The guidelines would be in the raspberry pi config.txt docs. As I'm sure has been mentioned previously.
You will have to read the main thread to find the location of this file as it is not in the same location as the raspberry pi images.
Honestly for your case use I would ditch the Linuxcnc image, install Linuxcnc via apt to a RaspiOS, there is information on the forum regarding this, or as I mentioned earlier build from source, you don't require an RT kernel to build from source.
The zswap fix is to actually have swap a compressed swap in memory. It is not just to "fix an error log message", which you don't seem to understand.
If you aren't going to bother to try I can't be fucked to give you help on what is really a non issue for an image created to run a CNC machine which is something you admitted you are not intending to do. And in that case you have been given suggestions to overcome what is a very minor issue.
Regarding the zswap it seems you didn't see or ignored the issue it caused for a user when the system was left idle. I'll repeat this one more time, it is not a fix for an error log message, the error log message pointed to an actual issue and the fix was to fix the actual issue.
So far I have offered some suggestions, yet you have not bothered to attempt any. Maybe this image is not suitable for your intended use and another solution maybe a better path to take.
Good luck in your endeavours but I have exhausted my patience and will to help