Hey
@E-DarkHorse,
I'll chime in with some thoughts. First, swapping CPUs basically shouldn't matter, as they're both socket AM5 and have similar requirements (170W TDP). Downgrading to a 7900 XTX should also be fine and will cost a lot less. Swapping the case is of course fine (assuming similar or larger size on the new case), and the same should be true of the PSU.
Motherboards generally don't matter too much as far as performance goes, unless there's a specific bug or if one of the boards doesn't properly support XMP (EXPO) for a given memory kit. I would personally be hesitant to buy a second tier motherboard (meaning, B650 chipset instead of X670/X670E) just because there are often features that get cut. As an example, I have seen boards where you don't get PCIe 5.0 support on the primary M.2 slot, just because it was cheaper that way.
You say at the end that you're not a gamer, so that makes me wonder what exactly you're planning on doing with this build. There are apps that will work better on AMD GPUs (certain professional apps), and the extra VRAM might help in a few specific scenarios. But if you're looking at AMD for AI workloads — unless you're specifically wanting to learn to do AI on AMD's hardware — I would strongly recommend Nvidia GPUs. If you're not doing anything super demanding, graphically speaking, I'm not sure why you'd even need the 7900 XTX. A lot of video processing should be just as fast on a 7900 XT or 7900 GRE, and probably as far down as 7700 XT. So that could potentially save several more hundred dollars.
For the memory buy links, unfortunately our pricing engine can screw up and link seemingly similar RAM kits (and other parts) based on whatever logic the programmers use. Or perhaps someone just screwed up and linked to a non-EXPO kit. XMP
should work but may not perform optimally. (TBH, most RAM kits beyond a certain point are mostly meaningless.) But I've put in a different kit (G.Skill Trident Z5 Neo) that should be better suited for an AM5 PC now.
Hope that helps!