Sorry @gprovost, didn't see your message until today. Anecdotally, as soon as I reenabled the automatic CPU governor, the Helios4 kernel panicked within 8 hours. Unfortunately I didn't have the serial console open to catch a trace.
Anyway, I rebooted my Helios4 with the Marvell XOR driver blacklisted. Nothing shows up under /proc/interrupts related to XOR now. Running a serial console again to hopefully get a trace if it crashes.
Unrelated to the crashes, but I think I ran into a bug with the Debian Buster armhf build of btrfs-progs 4.20.1. Trying to delete subvolumes o