Internal motherboard headers are to connect the motherboard to the front panel connectors. And I found it a right royal PITB figuring out what was what. USB 3.2 gen 1 = type A USB 3 port. You need USB 3.2 gen 2 if you buy a case with a USB C connector. I think.
If you're sure you won't be adding stuff later (and are possibly a bit ambivalent about building yourself anyway), I've been pretty impressed with the HP business desktop line - e.g. ProDesk or EliteDesk. They obviously go on sale too.
Or you could possibly split the difference with a PB Ryzen Business build? I've not used these myself though: https://www.pbtech.co.nz/product/WKSPB5350/PB-Executive-Series-5350-Desktop-PC-AMD-Ryzen-5-Pr



