So what's the best option for a new Primary drive in the 2-4TB market?
For primary, I'd go 1TB. Much better chance to score everything you need (TLC, DRAM) without breaking the bank. For storage/secondary, 4TB TLC DRAM-less. For the former, look for the best QD1 4k random reads you can find, for the former, it doesn't really matter (go for cheap). For either PCIe 4 is plenty, unless you backup very, very often or otherwise perform frequent massive transfers.