Oh hey this thread again. To answer PacketpunkUma: rebuild took like 14 hours for a 4TB mirror, and that was with the disk actually staying connected the whole time which it usually didn't
I ended up just moving the drives to an old InterServer micro tower I got secondhand. Same power draw basically, way fewer headaches. The ARM box is now a pi-hole and that's all it does
Still think a SATA hat might have saved it but those cost almost as much as the used x86 box did