Putting these two together, I'd say you wouldn't be unhappy with a 4" 6-shot service-sized .357mag revolver with adjustable sights. It doesn't get much more versatile than that. A great range gun, it can be carried when need be. It's very versatile in the ammo it'll shoot - It can shoot low-power target rounds or hotter .357mag - and handloading for it is pretty straightforward. The adjustable sights make for a nice sight picture for target work to work on your fundamentals
*. You can shoot it single- or double-action, and in 6-shot, there are good speedloader options available, and you can shoot it in most matches if you even get the itch. Most have numerous aftermarket grips available, and most are easily worked on if the action could use some work. And, finally, IMO, learn to shoot it well (in SA
and DA, please), and the transition to semi-autos is generally quite easy. The reverse doesn't always hold, from what I've seen.
* I'd be remiss if I didn't point out the obvious - developing solid fundamentals is a terrific goal (one that I heartily endorse), but it's more about your commitment to developing solid fundamentals than it is about the gear. It's true the 9mm round isn't commonly thought of as a target round, and in a polymer gun, even less so, but your commitment towards getting better is what really leads to getting better. 'Course, a nice trigger, good sights and good wadcutter ammo are big helps...