Would a TB-PCIe enclosure like the Sonnet ones not cut it? I get that it's still easier to have PCIe built in, but that's a big premium to pay.

Those cost hundreds of dollars and often only have 1 slot. The premium can’t be avoided.

Yeah, but it's either that or buying outdated hardware at this point because they don't refresh the Mac Pro. Idk why Apple didn't just make their own expansion add-on.