The RAM is fine (Memtest ran 4 times without faults), and cooling seems to work well enough. Storage is ok and I used two different SSDs through this whole process and saw the same problems on both.
I tried the previous known-good kernel options on the Manjaro install and it seems to be OK now. According to the Arch Wiki the Intel 8th Gen mobile CPUs and especially iGPUs are known to be a little problematic on Linux so the kernel options to disable some power saving options are basically non-optional. It’s weird though that it works now and didn’t on the Tumbleweed reinstall.