That is a dilema! My dog doesn’t like having his harness put on either and is very uncooperative but we‘re not quite at your stage yet, it must be very frustrating. You seem to have already considered most of the things that come immediately to mind and the problem is once they‘ve associated it with something unpleasant, its a bit of a battle to turn it around again.
This might sound daft and a bit of a long shot but worth a mention anyway
- when we have the heating on and the air is particularly dry, we seem to get a lot of static, - I‘ve had a „shock“ when Humphrey‘s touched me and it has also happened when I‘ve been putting the harness on, which could perhaps have put him off it I suppose. Another thing is, he seems to better if I approach him with the harness from the side or behind, I think they dislike it when we loom over them head on - particularly if eyesight is failing, that might make them nervous, Humphrey‘s better if I‘m at his level.
The only other thing I can suggest is working with treats, hold the treat so he has to put his head through the harness to get it and then bribe him some more until you‘ve got it completely on, this would probably only work for aleviating a bad association though but not if the cause lies elsewhere.
Don‘t know if that helps, you‘ve obviously given it a lot of thought already but I can’t think of anything else....Hope you can sort it, best of luck!