Yeah I have found some cards that don't require an external power connector so I imagine that is a good idea that it uses less power. The one that's in there does.
As a side note here's a quick video I threw together of what happens:
https://www.youtube.com/watch?v=WxdBbEGc8Ps
That boot issue is completely eliminated by removing the add on card. I have tried different cards and all had the same issue, and the PSU is 600w so it should be enough to run a single card.
For now I will run with one screen and try to see if I can get Synergy or some other form of cross system multi monitor going - that was actually my original goal as I hate how OSes (mis)handle multi monitor and dialogs/windows tend to open all over the place. I can't afford to buy a 4k monitor right now, but should be able to by Christmas time so I'll probably just wait for the deals to come out, if I can't figure out Synergy or an alternative.
Something is going on there. I saw you trying to update the BIOS and upon restart it hangs, but did the updated BIOS actually stick? If not, I'd remove the video card and try updating the BIOS without it installed to see if the BIOS update works. If it does, make sure to load optimized defaults.
So it could be the motherboard that is the issue, or it could be the video card. The only way I can think of to know for sure is to install a different card in the system, and see if the issues persist. If they do, you know it's the motherboard. I upgraded my oldest son's computer this past winter, and bought a H97 Gigabyte Ultra Durable line motherboard from a retailer. The first one arrived poorly packaged (flat-rate box, no padding), and the box was was crushed. I didn't see any damage to the motherboard, so I tried it out. I couldn't get the system to be stable, so I returned it and got another one. The second one came with good packaging, but it was unstable. Weird crashes, hangs, and there was something obviously wrong. I keep extra components, so I ended up trying different RAM, CPU, video card, PSU, and a hard drive. It still was unstable. No dump files, but it would just crash and reboot.
So I looked online, and found the TonyMac computer forum (building computers with using the Mac OS), and that was a board that could be used to build the Mac clone. There I saw user after user complaints about system instability, and nobody could figure it out. Most said screw it and replaced it with another brand of motherboard that would work for them. After seeing those posts, I boxed it up, sent it back, and bought an Asus Z97 motherboard, and I never had one issue with it.
I have used Gigabyte boards on many builds over the years, and that was the only time outside of one Abit motherboard I bought in 2000, that I could not get it to work properly. So like all manufacturers, they all occasionally produce a turd. Maybe that motherboard is in the same category as the one I couldn't get to work.