Dealing with potentially bad response from supervisor upon leaving with MSc for different PhD program
This is all opinion, so it is only worth what you pay for it, of course. But I would first consider how long it will take you to finish your current degree. It is pretty miserable being miserable for three more years, but if it can be done in a year then it might be best to just tough it out. 

There is no reason, in principle, why you can't change fields later, either gradually, from a paid position, or abruptly. If the fields are different enough you may need credentials there, of course. 

If you do complete the doctorate, then your old advisor's opinion about switching won't be of concern, only his/her opinion about the quality of your work. 

However, another alternative, if you have picked out a different university, is to quietly explore with them the possibility of switching. If you can find a compatible potential supervisor there, the current situation may not be a detriment to switching. Personal contact is probably best here as it enables you to make judgements about how much to reveal about the current dilemma. 

But if completion is a long way off, then quality of life would seem important enough to make a break, even if not all of the consequences are positive. But only you can balance the forces.