The support hand runs the new mag in. I must have short thumbs. Here's two pictures, one with a Glock 19 and the other with a Sig 250. See how my thumb easily engages the slide release on the sig but just bare touches the slide release on the Glock.
I think you are needlessly eliminating quality firearms from consideration.
As primarily a 1911 guy, who spends a lot of time on 1911-specific forums, there are always threads from new guys looking for extended slide stops, extended mag releases, and grip panels with scoops on the mag release side, because they can't reach the slide stop or mag release with their right hand in a shooting grip.
I believe that those things are intentionally put out of your reach to avoid fouling the gun while shooting.
The things you should be touching while shooting the gun are within your reach, like the trigger, and on a 1911, the thumb safety. The things you don't want activated while shooting, like the slide stop or mag release, should be out of your reach.
Shannon Smith on mag changes with a Glock. You don't have to watch the whole video, but note his mag release comment in the 2:05 - 2:20 mark, and how he likes to drop the slide at the 7:45 - 8:05 mark.