Make the sdcard a read only root fs, problem solved

Doesn't actually help. It eventually goes corrupt anyway. It might be related to power ripple or something.

Makes it easy to replace, though.

Could be some other hardware issue for you, yeah. I recall there are some problems that can happen if the usb power supply is too weak. But this fixed it for me.

Yeah that's more or less my goal for the setup in the post... I didn't have much time to investigate what else I could move off of the SD card but I suspect there's a few other things I could do. NixOS makes it super easy too.