Just want to post a post mortem for this build for anyone looking at this forum.
1. The GTX 680 works OOB no problem whatsoever. I switched to a 780ti and there is the issue with the web drivers not being always up to date so you have to always, ALWAYS wait for them to be updated after a system macOS update.
2. Wifi/BT work with the used m.2 card, however, not OOB. I had to edit some Clover entries, look a forums, pain. Now, everything works (except one major feature).
3. The system is stable, but I have problems with audio/sleep. The audio problem can be easily circumvented by using a USB sound card. I use the Creative SOUND BLASTER Play! 2 and I've had no problems whatsoever. Don't bother with the built-in audio, it's a pain. But sleep, meh. Can't get it to work, I am always shutting down the computer :/
PROBLEMS:
Sleep not working. I sometimes can wake my display/computer, sometimes not. It's infuriating.
Handoff/continuity/Apple Watch unlock/Universal clipboard - not working reliably from the iPhone/iPad to the Mac. It works 90% the other way around. Apple Watch unlock doesn't work at all. Is is my main complaint. I reallly expected this to work, since I was used to this on my iMac and now my MacBooks. Don't buy the wifi/by card I used, try a different solution.