please note that Leopold has produced the FC750R in at least 2 variants with differences in the PCB, Plate and spacebar stabilizer spacing. One of these 2 variants (the one with the stabilizers closer to the spacebar switch) does not support GMK / Cherry and IMSTO standard spacebars.
The one with the standard stabilizer spacing has FC750RT marked on the PCB:
i have an fc750r and a mac.
function / media keys can be a little 'wonky' but,
switching alts for commands and vice versa got it to work through karabiner https://pqrs.org/osx/karabiner/