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 on my 17T btrfs volume is impossible-- btrfs subvolume delete gives me "ERROR: Could not statfs: Value too large for defined data type". I found a post from someone using Raspbian buster over on the Raspberry Pi forums with the same issue with large btrfs volumes: https://www.raspberrypi.org/forums/viewtopic.php?t=249873
I haven't tried btrfs-progs 4.7.3 from Stretch as they did to see if the problem persists, yet. If anyone else with a large array in a Helios4 with Debian Buster and a large btrfs volume could try creating and deleting a subvolume, I'd appreciate it. I'm assuming this would have to be filed as an Armbian bug report, or possibly upstream??