Losing altitude in a turn is something you prevent by using up elevator, not by trying to make the turn flatter.

With a model there's no precise way to tell whether your turn is coordinated, because you have no turn-and-bank indicator and you can't feel the seat pushing on one side of your rear end. But you can be sure that if you are cross-controlled your turn isn't coordinated. I get the sense that most of your difficulties come from your apparent belief that flatter turns are desirable. They aren't, except as a way of having fun by making the plane do something that planes don't normally do. Banking is the essence of turning: it makes the lift vector point to the side, and that is what makes the plane turn. The rudder is there mostly to prevent adverse yaw, as Rob2160 explained earlier. It isn't the primary turn control. Even a plane with no ailerons normally turns by banking, because the rudder induces yaw, which, because of the dihedral, makes the plane bank.