It depends on your bandwidth needs. I also want to build the thing you want. There are no multi SSD M2 TB5 bays. I made one that holds 4 drives (16TB) at TB3 a

It depends on your bandwidth needs. I also want to build the thing you want. There are no multi SSD M2 TB5 bays. I made one that holds 4 drives (16TB) at TB3 and even there the underlying drives are far faster than the cable. My stuff is in OWC Express 4M2.