When I’ve needed to do it, I think that rather than create separate maps I’ve always merged the files that I’ve created into one before running splitter and mkgmap. I’ve not needed to do it for a while though - it’s rare that I visit non-contiguous countries!
Consequently I’ve never worried about the family ID et al because I’ve only ever got one user-created map to worry about. https://www.openstreetmap.org/user/SomeoneElse/diary/38613 is something I wrote a while back; while that’s not exactly what I run on a monthly basis I think it’s close. https://github.com/SomeoneElseOSM/mkgmap_style_ajt/blob/master/garmin_map_etrex.sh is similar.
