What a great description!
Yes, that co-incides with my experience, and is the best description of Filco SA, ever.
They are among the nicest keycaps in existence, but it all comes down to how you deal with the profile. Personally, if these were in a sculpt like in SA Pulse, maybe I’d actually use them. In R3, they turned me off SA profile, but some of those sculpted SA clones on AliExpress were pretty nice, so I think that it is the R3 that let down these keycaps.
They are thick and semi-polished. Which, as you note, does MX Brown no favours! I also agree that nice linears would be good with these. Something like Momoka Frog on the cheap end, or a mid-weight L + F V2 Durock linear. I bet the R1 HHHH linears would be nice with these.
Yeah, I just wish they had sculpted these, then I could put them on a 104-key with fictional Cherry MX Yellow linears.