I thought I'd wade back in after sending my R5ii into Canon for a second time. Again, they replaced the printed circuit board, and checked the sensor, firmware, etc. It arrived just before heading back to Churchill for another season of photographing polar bears. For the most part, the new mother board seemed to work well. The camera didn't crap out (totally), and the battery level didn't drop super fast. On the other hand, it wasn't as cold as last year so I may yet see a return to previous challenges. What did happen on a regular basis is the dreaded dead battery alert - however popping it out and back in while the switch was in the on position, got the camera up and running again. I often shot between 350 and 480 frames as well as a fair bit of video each day and almost never changed battery.
A colleage believes the huge venting on the mark ii has something to do with it so he tapes his up in the winter and that seemed to help in his case.
I've spent a ton of money to avoid the problem and I hate throwing more money but I can't risk a camera dying on my photo tours. I've bought a dummy battry to run off a Dtap if necessary as well as a second R5 mark i body (since I know that seems to work in all temps). In January, I head back up when it drops to as low as -50C/-58F. My old 5Ds laughed at those temps, and my colleague has been using his R5 in those temps for several years. Fingers crossed the mark ii still works. I'd like to use my R5 for time lapses and my mark ii for photos.