I think one can always improve something about the voice
Old habits are really annoying, but I am not so sure SLT can help with much more than maybe recognizing them - they will have to go away by always watching them and avoiding them if one falls into them. For example if I argue with a friend, my voice tends to get lower and louder, this is not good for my voice...
My SLT did mostly non trans related stuff with me post OP - so basically it seems that one doe snot realla always need a trans specialist post OP to improve the voice. We just mainly did voice health exercises - in a way, when I use my voice ow properly without hurting or distorting it, its female anyways, so no need to feminize it now. Just to not misuse it again.
The same may be true for resonance - it may be that it is less about forcing the resonance into a female resonance pattern now, but basically to just get resonance at all, or rather projection and loudness - the male way of getting this does not work properly anymore, it hurts and sounds weak and broken, so one basically again just has to learn to use the voice in the way it works now.