>>105601125
>iSCSI
look also into nvme-of which is a better alternative
>10G ethernet directly to my PC for iSCSI
you can go 25, 40 or even 100 using cheap used mellanox connectx cards
>I've got an Asus Prime X570-Pro mainboard on hand, but I assume that it does not have enough PCIe lanes for everything and I don't know if I can even bifurcate every PCIe port
it supports x4/x4/x4/x4 and it seems it can also do x8+x4x/4 in the first two slots

I'm in the same boat as you but thinking of going deeper (something like 4 nvme raid, optane and 40gbps or 100gbps network) so I've already looked into the cheapest way to get more pcie lanes
you can either buy an old lenovo thinkstation p520 (5 pcie 3.0 slots, 48 lanes) or build a x399 gen1/2 threadripper system (3-4 pcie 3.0 slots, 48 lanes)
there's also cheap 1st gen epyc combos (search H11SSL-i in ebay), that will have like 6 pci slots and over 64 lanes