I've looked at the vangs. I'd want to try it out on a loaner first if I could then look at vangs. I also need to estimate what line I would need for a 4:1 - six times the length from the anchor point to the top of the davit at it's furthest point plus enough feet for handling.
I noted a friend having issues with his manual turning hoist, similar to yours, this summer. As he ages it just keeps getting harder. A power driven rotation would be so much easier. I understand how pulling the dinghy up that hill gets old pretty quickly. I also understand the firm NO you get from Sian.
You could use a light tackle pulling from the D ring, just to get the top of the boom inboard enough to mostly right the boat. You just need to get it far enough inboard so the weight of the dinghy is not causing more than a degree or so of outboard list, then pulling on the dinghy will do the rest.
I think your AC winch is a bad idea. She won't use it. You will end up with your dinghy being towed, if used at all.
I have an unused AC winch. Originally from Harbor freight, cost me little and powerful enough for the job.
I wanted to hoist my dinghy with less effort. My tackle on the stern end was 4:1, on the bow, 3:1. The bow weighs ~250#, stern ~500#.
The logistics and aesthetics of installation of the AC winch defeated that exercise. I now have 6:1 on the stern and 3:1 on the bow and a season in, am satisfied that I can continue to lift using sailboat winches and that tackle for several more years.
For the price of the AC winch you can get good looking sailboat blocks and some nice looking line that will be more than adequate.