I’m assuming you want to do DIY and have 10G networking, as this lets you take full advantage of the speed of the SSDs, but there are some pretty decent options from Synology if you just need a NAS. Mostly, just what is your reasoning for going for the 2x high capacity SATA SSDs?
For motherboards, you’d want to go with either a Mini-ITX or Mini-STX server board. First that comes to mind is if you just need a NAS, then the SuperMicro X10SDV, X11SDV, or X12SDV TLN4F sku boards as they are Mini-ITX, have Intel 10G NICs, full size ECC RDIMM support, and Out of Band Management. If you need better CPU horsepower, then the AsRock Rack X570D4I-2T would be my pick, as this also has intel 10G NICs and Out of Band Management, along with 2x Occulink connectors for either 1x U.2 NVMe SSD or 4x SATA drives from each connector at their full bandwidth. Only caveat is that the power input is 12V only (it comes with a 20-pin adapter, so its all good), the mounting holes are LGA1155, and you’ll want just a bit of airflow going over the intel heatsink to keep it from blowing up your logs with temp alerts
Velka 3, K39, or J-Hack Pure X are some decent options for a SFF case. I’ve built in the K39 and Pure X and both are pretty great options. I like these as they use Flex ATX PSUs, so don’t need to deal with having a bulky power brick. Highly recommend if you go this route though to do a fanswap of the Flex PSUs fan with a Noctua A4x10
Here’s some shots of a K39 build. 2nd picture you can easily place the 2.5" SSDs below the GPU along the bottom left of the case:
And here’s some of the J-Hack Pure X. For this one, I’d probably use double-sided tape to stick the SSDs to the side walls on the power-button side of the case. For reference, the fans on the sidewalls were not necessary, I moreso wanted to test if I could and didn’t stop to think if I should XD: