If you are second-guessing about the CPU, you can’t go wrong with the 7800X3D. It is more expensive yes, but also a beast. With that you don’t have to upgrade for many years.
You have 64 GBs of RAM. 32GBs i more than enough, unless you do heavy video editing or other reaaally professional stuff. You should take 32GBs and use that money for better CPU or GPU.
I have been using Nvidia for a long time and haven’t had time to get into AMD scene, so I really can’t give any input on your choice of GPU.
Water cooling is not necessarily better than air cooling. Check out this video by JayzTwoCents for input: https://www.youtube.com/watch?v=RQ3c0kspOb8
Be sure to buy a fan for rear exhaust! I didn’t see one preinstalled in the case.
And lastly about the monitor. You probably could get a cheaper FreeSync monitor. Be sure to check extensive reviews that test all aspects of the monitor. That means no reviews from PC Gamer etc.
Those mATX’s are usually cheaper than ATX ones, since those don’t have as many slots for cards.
Could also be that this particular board has features that OP didn’t find on an ATX board, like WiFi support.