I don't use GPS on this quad, so that's not an issue - I just ripped 3 packs an hour ago, and it's working pretty good on the default tune, no failsafe, so it has to be an issue specifically with this FC and Crossfire - I'm just glad I got it sorted out

I see that you got a response from a Betaflight dev to your reported issue (HERE). You already tried UART6 and the V6.17 Crossfire firmware so it's odd that your board isn't working but it seems to be working fine for the Betaflight dev. As yours is working under INAV and also the Betaflight 4.3.0 RC5 release it can only be a Betaflight firmware issue with RC6 and RC7. I would post a response requesting that the dev who replied to you supply a copy of their working "diff all" so you can compare it with yours.