So yeah, seems like it's a memory issue, thanks to @KellyBundy for the suggestion that I run MemTest86.

There were so many errors in the test that it just gave up when it hit 100000, which is odd, because the system boots just fine and I've never had a problem with crashes (hence why I didn't immediately suspect a hardware problem). Even the simulations run fine (usually) until they reach a certain size. But the memory test was showing a multitude of single-bit errors, always in the first two bytes. I'm not that experienced with this kind of problem, but I tested each of the four modules in each of the four DIMM slots individually, and they all failed all the time, so I think it's probably either a PSU problem or a bad memory controller on the CPU, but until I can find a known-good PSU to swap in, I won't know which (I don't have access to a PSU tester). For reference, there's 128GB of non-ECC UDIMM, which in hindsight may have been a little ambitious. The CPU is a Ryzen 9 3900X.