I really like your stuff and I wish you would finish them more.
...I'm trying to practice composition methods simple enough that I would be comfortable making animated game sprites with, not so much to create images that stand on their own.
I completly agreee with that, and I think this is a perfect composition for that, but dont let this become just an excuse you used to counter helm in this topic, start animating this tank, now.
Dont put it off, if it seems so hard you have to put it off your simplicity focused composition is all wrong, start animating it, start with basic colors and as few outlines as you can, then build up to this but dont let the inspiration that drew you to make the tank halt before you've started animating.
if there are any changes you should do to the model, it would be simplifying it,so forget about extra shades, posters and Helm's compromises.Eliminate as many outlines as you can, if an outline isnt necesary to describe a shape, if it's there for style then get rid of it, they do nothing more than geting in your way when you animate
when you can see how many frames you can create, and how tedious it is to shade each one of them, that is when you have the judging elements to decide wether or not you need more detail.