I've removed quite a few chucks from hand drills. They have all been
threaded on, with a keeper screw.

The keeper is left handed, as you know, but the chuck is right handed.
They have all come off with the hex-key-in-the-jaws,hit-with-hammer
technique. A few have been quite stubborn, but with the drill on a
solid surface & a smart whack, they have all come off. I don't think
that I've ever had to use anything bigger that a 16 oz hammer.

My intuition is that a smaller hammer, moving faster, would work better
than a bigger, slower, hammer.

HTH,
Bob