Hey Lucas, to be honest the best drills for this are just getting the other player to mix up the serves to you and try to make the right choice and play a good return, you can ay the point out of you want to see how your returns are played by the opponent. It’s difficult to practice serve returns with multi ball as feeding it like that won’t be e the same feeling as returning a real serve.
But definitely getting them to mix up the topspin and float serves which you are struggling with and to the middle line too. With those I would say always try to move and play a forehand as this is easier and you are more flexible from that position.
Dan 🙂