usb-c for data transfer is working nicely, and I'm getting a lot better at hot air soldering!
Unfortunately, the mosfets I ordered were the wrong size (too small by a factor of like 3). TIL that sot-23 is not a size but a shape that can have different scales. This doesn't stop development or testing, but prevents the battery reading indicator and the new power saving around that from being tested for now.
When the correct sized mosfets arrive I'll assemble another for hooking up to the Current Ranger and see how standby current has changed.
Also in this new revision, I've wired the display polarity refresh to the samd21. I'm going to experiment with mapping this directly to the internal clock with a divider to refresh it at somewhere between 1-30 hz. Then I can ignore keeping track of this in software during standby - in theory. The display supports switching polarity via SPI commands, but not both modes at the same time.