All the PC parts should be compatible on paper but if there is something standing out please let me know. Or if there is a better alternative that I am not aware of, I would love to hear your thoughts. Thanks!
PS. The 6750xt was $379 just yesterday :( Missed my window.
If it's a gaming computer - loading times will be faster. Generally speaking, doubling the rate of an SSD in such a case will provide more tangible benefits than faster RAM or faster CPU. You wouldn't notice a few FPS difference in a game, but you would notice it loading quicker. In all seriousness, if Zen 4 doesn't support DDR4 as you say - maybe going with an older gen CPU is a better value for money. I'm sure the performance advantages are not significant (they usually aren't between subsequent generations), while the savings are - and they can be used to upgrade other parts while staying under budget. I suggested storage, but FWIW this money can just as well go towards better peripherals or a screen.