Random segfaulting is not something that “just happens” because of an OS misconfiguration, then if the same problem happens on Arch as well as on a clean EndeavourOS live image it convinces me that it is in fact hardware related somehow. As you have already replaced the RAM, my guess is CPU or motherboard issue.
Zen2/B450 is a widely used and well supported configuration on Linux that you normally shouldn’t have issues with, but Zen2 CPUs are rather notorious for having fragile memory controllers, and sometimes dodgy AGESA firmware releases that can cause issues on some CPUs. I used to have a 3600X myself that started crashing at idle around a particular firmware release of my motherboard, and it was fixed by a subsequent release.
BTW the fact that it doesn’t happen on Debian doesn’t necessarily mean that Arch is the culprit. It could just be that Debian is not triggering the fault because of different, perhaps more conservative, compiler optimizations.
As a last ditch effort, you could try resetting your entire UEFI (bios) settings to default, preferably by pulling the CMOS battery.
BTW, is it only GUI applications that are segfaulting? Or other programs as well? Do you have an old spare GPU you can test with?