Let me ask this, how much for the 6800? If not a lot more than the 6750, maybe check into the rx 6800 and a ryzen 5 5500.
Especially if you go with a ryzen 3600 or 5500 I wouldn’t worry a lot about switching the b350 motherboard. I actually ran a 5900x on one for a while.
To be honest on a budget I’d go with the 3600 or a 5500 and better gpu. I don’t think you’ll see a lot of difference on pci e 3 vs pci e 4. Even if even if you do I don’t know that you’ll see enough cpu performance uplift to justify a new board as well. So I’d consider going with a slightly cheaper cpu and get the better gpu which for most games will be your big factor anyway. Most of the stuff I’ve seen, pci e 3 is slightly behind pci e 4, but not by a high enough margin at that tier to be more than a few fps in most cases.