First, I want to ask you if you use a good piano bench (adjustable height) or if you have a low-budget solution. I have seen many videos where the player has been sitting on desk chairs, kitchen chair and even on beds (!) ... obviously because they have so little space in their rooms, but it hurts just to watch ...
Then, I suggest that you just try to play a chord, with one or both hands, and then you stay there, sit still with the keys pressed down. THIS should be your most relaxed position! If you feel tension while you try to maintain this position, you are definitely having wrong posture. For example, do you feel that you must continue pressing to stay in that position? Do you feel tension in your body?
Again, this is your resting point. Then, if you are to move from it, to another chord, you must of course make an effort to lift your hand and arm and move to next position, but then - boom, back to rest again.
I also believe most of us get a bit tense when we play something difficult and know that THIS particularly hard passage is awaiting around the corner. In worst case we get nervous about it several bars in beforehand! How can you avoid tensing up (and shoulder tension is the major symptom) in such situations? Well, I recommend backward chaining for that. Start by playing the last note of that difficult passage, then add the second last and so on. You will get pretty tired of that last note after a while, but the real big advantage is that it gets very familiar too. Finally it becomes your "grounding point", the place where you land = your new resting point.
And in that way you will get more and more relaxed the closer you come to this point, instead of the opposite ...
I am no expert or piano teacher or anything, I just write from my own limited experience. To me this works, at least.