There are radios that retain settings when off. (Probably built in battery). But off hand I can't name models. Maybe the fusion but I'm not sure
Or as above 24v radio would solve it but I imagine would have limited options.
there isn't really a good way otherwise. Other the leaving the converter always on and switching only the outputs (except the stereo memory). But then you have a decent size draw all the time running the converter.
Another option might be a smal. ~10a 24-12v converter and connect direct from batter to mem wire. Might have less standby draw then your large 24-12v converter.
If you are going to put in a battery I would put in a 12v bank and move all 12v to it , remove converter, ad get a 24-12v charger