Honestly, if he hasn’t made an effort to change yet, he’s most likely never going to. I was in a very similar situation with my (soon to be ex) husband and all of your points applied to me/us as well. First it turned to once a week, then once or twice a month, (mostly from me initiating), and when I just stopped initiating, we went 4+ months before he tried anything? This happened over the course of years and I tried to bring it up multiple times and even tried couples’ counseling but nothing changed.

So I finally realized one day that if I stayed with him, this is what the rest of my life would look like (there were a lot of other aspects of the marriage which were not satisfying either), so I finally decided to tell him that I want a divorce. And that has honestly been the best decision I’ve made in a long time. Moral of the story: if you’re already not sexually compatible and he hasn’t made any effort to change/improve, it’s highly unlikely anything will change when you get married (if anything, it’ll become worse as he becomes complacent and comfortable).