That's a good point to use one loop to manage all units indeed. For collision, non oriented box to box collision is really enought (this is not precise fighting game collisions !)
That's lot more faster than math sphere calculations.

For 3D models, and sprites you should have pre calculated sprites indeed, and not calculate them, that's better performance indeed, and you'll boost a lot the speed using sprites.
(this is 3DGS ,not a AAA speciallized RTS 3D engine that can run thousand of units)