Got another attempt at M81 last night. Seems like the issue with monochrome camera has been resolved by overhauling the wiring on the my rig (it was likely a power issue of some kind).
Did shorter exposures but for a longer integration time (240x120s = 8hr). I can barely tell any difference from my previous attempt (46x300s = 3.8hr), and most of the difference is probably just due to different processing.
I think I'm just limited by sky glow and camera noise at this point.