If the ergo kit is as specified, and you want to support boards like the Boardwalk, we need 1.25U x 2 on R4. Personally, as a Boardwalk and Planck user, I'd kinda prefer it if there were base alphas + separate kits, but I can appreciate why you haven't done that.
At least if I get a base kit + an ergo kit in this setup I would have enough to cover both the Boardwalk and a Planck, haha.