My servers in production are Broadwell or Haswell Xeons, either Dell R630s or Lenovo RD550s. They have crap tons of RAM and most of them are SMP, ~2.4GHz/core systems that are perfectly adequate for the load. They're old, old enough to be E-waste for corporate systems, but they're also perfectly fine except for storage needs.
These machines don't have U.2 and many of them don't have any open PCI slots. The Intel S4500 drives I use were new in 2018 and are some of the last high endurance 2.5" SATA drives made (note: I have cold spares; I'm not going to run out of those drives anytime soon). I can't remember if they're MLC or TLC off the top of my head, but they're good for .95DWPD. If there are any spinning drives in those same machines, they're also no bigger than 4TB; Seagate makes 5TB 2.5 drives, but they're SMR and screw that for anything in production. Anything bigger than 4TB in 2.5" SATA or for that matter nVMe is probably on crummy NAND.
The combination of drive density (only 8 2.5" drives per chassis) and capacity is becoming an issue. I don't really want to buy a new server just to get some U.2/U.3/EDSFF, but it's looking like I either have buy something new enough to support better drives or I have to homebrew something. Preferably something 1U, because if I use more than that I get kicked up to the next tier of pricing.
I'm tempted to put together a modest system with a 10GbE NIC, an extra 8 or 12 of those 4TB Intel S4500s and HOPEFULLY find room inside a 1U chassis for a 4x U.2HBA so I can have some larger drives as well. I guess they'd probably just sit loose inside the chassis. Whatever.. Set the whole thing up as an iSCSI target.
What I do know is that nobody is selling a generic chassis with better-than-SAS drive connectivity, and in the land of affordable NAS, the trend is to use either nonstandard drives (thanks, Synology) or shitty consumer SSDs. I'm assuming I can find a small motherboard with 10GbE integrated and 2x 16xPCIe slots. That's probably a tall order, but I'll bet it's still cheaper than trying to deal with a new, entry level 1U system from a tier-1 OEM.