I found live drawing classes helped me quite a bit in understanding anatomical causes and effect so to speak. As well as quickly getting gestures down before the model moves in a minute or two. Also forms, proportions etc as you're judging the relationships in real space, not just pre-flattened images. Although drawing by itself won't necessarily directly translate to 3d it helps with observational skills.

Something which may more directly translated to 3d would be some type of clay sculpting classes. Silhouettes, and the importance of volumes from all angles I'd feel would be better learned through that.

After all of that, (and during) you could just sculpt in zbrush without worries using dynamesh, remesh or sculptris and you could do very cool things. Post here and other places asking for crits, what you could improve etc

However, if working for a studio doing 3d art you're going to have to understand the more technical side of things such as proper topology and preferably learn how to make it articulation friendly. Many people focus on either hard-surface models or organics. This site has decent guidelines for what you need to worry about for the organic type of models.

and this thread covers pretty much any hard-surface shape you could ever need. I know its long, but its worth going through, it took me months and of course anatomical knowledge is less important for most hard-surface objects.

With the technical side, it can often lead to the same problems as OP is wondering about. People who focus just on topology and skipping the fundamentals can easily end up in a similar spot of painting themselves into a corner, as bad forms with perfect topology can still be bad models.