Update: Today was my 4th day with my rental RP and I'm happy to report that after evaluating it all weekend... I bought one! This camera is awesome and really exceeded my expectations. It has to be one of the best values in cameras today, a mirrorless full-frame for under $1,000 with EF adapter included. That's just amazing. It may not be the camera for everyone, but for me it's got everything I need. Depending on my "portability" and IQ needs, I can happily bounce back and forth between the RP and the M6 to fit what I'm doing for the day. The new RP should be here Tuesday, right when my rental goes back, and I look forward to getting the new one all set up.
I'm probably going to solve this particular conundrum by buying both.
I DID just buy an M6 mk II.