I see you are living my life
At this point though, I'd definitely keep a lookout for something that can do nvme on all drive bays (I think the R740xd can...?). Enterprise SAS and SATA ssds aren't coming into the secondary market nearly as much as NVME 2.5 drives are nowadays and when they do, they are priced a lot higher.
Might mean a little more cost at the outset, but you should pretty quickly make it back when buying your SSDs. And make it back again whenever you hit the point of needing to upgrade SSD capacities