I downgraded the BIOS to F2, and even re-enabled EXPO and I have not had a crash since. I will try upgrading to F3a this week as that one contains a fix for the Sinkclose vulnerability.
I don’t think at this point that the GPU was bad, as I got a crash even with my old GPU put in. When I built this system I was just waiting on the GPU, so before this it never had a GPU in it, I was just using integrated graphics. I think somehow having a GPU installed triggered the BIOS problems with F3j. It also seems to me that putting the GPU under load would trigger any underlying issues more, but that was never the case. I never got a crash while running games, benchmarks, or LLMs. It was always on the desktop, or occasionally in the launch or loading screens of a game.
I will give it a few more days to be sure and then mark @soundofthunder’s comment about the BIOS as the solution. I appreciate everyone’s help, even though it didn’t end up being a Manjaro issue. I’ll add that I’ve been using Manjaro as a daily driver for about 3 years now and have had very few problems.