When do you plan on building these systems? Waiting for Ryzen 3000 Threadripper (supposedly October) may be your best bet as they will be more likely to have built-in 10GbE on smaller board(s) (assuming there is a microATX variant, which may not be a given) as well as the higher core/thread counts (rumoured 48 to 64 cores at high-end). Otherwise, looking at built-in 10Gbps would lead to looking at ATX or larger motherboards (from either AMD or Intel, as I don't see any micro ATX or mini ITX boards with 10Gbps natively, at least not on Newegg), which you can still build compact-ish systems (e.g. with a Cerberus X case) with but not as small as you could with an ITX or DTX system. AMD would likely be ideal regardless as you currently get more bang-for-your-buck (cheaper per thread), generally more PCI-E lanes, and support for ECC memory on consumer-grade rather than having to shell out for server-class parts.
If you don't need built-in 10GbE networking (you can add it in via PCI-E cards), then you have considerably more options for true SFF, as mini ITX AMD AM4 boards should meet your requirements. PCI-E bifurcation should allow you to use a dedicated GPU and an add-in 10GbE card (though I haven't done bifurcation myself yet so you may want to check with others about whether it works well; there are some threads here on the subject). Ryzen 3000 CPUs currently go up to 12 cores, 24 threads with the 3900X, with the 16-core, 32-thread 3950X coming out in September, and with IPC and overall performance very competitive with Intel (or outright beating it, depending on application). Power at idle is low for all these chips, and relatively power efficient at load.
Two SSDs is easily doable on pretty much anything nowadays. My current build-in-progress is using the new Gigabyte x570 ITX board, which has 2 M.2 slots (not all ITX boards do). Saves space and simplifies things to go all M.2, though you can also add more drives via SATA.
As for the case, you can determine what to get depending on what parts you want to go with. There are SFF cases that will accommodate 2080Tis (if you are running workloads that can take advantage of GPU compute, otherwise a "mini" card may be ideal), though obviously the smaller volume cases will be more restricted in GPU length or slot size; you'll want to double and triple-check compatibility before ordering everything. There are some really good case options from the vendors and boutique case makers here, but you can also go for an off-the-shelf option if compatibility allows.
