For 4-6 rust spinners one lane is fine and nvme to sata seems like a nobrainer.. no sane zfs layout will bottleneck you
I personally run x710-da4 (4x sfp+) in the x4 slot in one box and Mellanox connect-x 4 in another both runs without issues
If you only plan to use a max of 4 spinners, then u can go nvme to sata, and get a 4x nvme adaptor for the x16 and not x4x4x8
Since i run my truenas baremetal i can also use the onboard sata for a total of 14 sata ports, with a cheap LSi 9207-8I
I've tested several newer models of lsi but keeps comming back to the 2008 based cards for pure tested stability (if only sata drives)
if you run newer sas then as minimun go 9300 serries based on 3008 chip