Problem with a 50mm on a crop is that you have to get too close to the target and they see you ( fine for babies ) but once you get to 70mm sometimes the house is too small to get far enough away. The 70-200 (40-150 mirrorless) buys you a lot of flexibility in that regard but is really better suited to a full frame. The sigma crop is real interesting but you need image stabilization (at long reach)nd it doesn't have it.
I could easily and happily replace the big whitey with an 85 and a 200, but that would cost me more dough and not less. The zoom is mostly useful to me for sneak pics and at ball games chasing the kiddos.