I have a sideways leaning tree planted years ago at my parents place. It's over a decade old now and gives us lots of apples. We don't prune it, and leaning sideways doesn't hurt it.
I try to plant my trees better now so they'll be less likely to lean in the future, but I wouldn't cut down or over prune to "correct" that. It's cosmetic in my opinion.
I wouldn't cut down a tree that might have some disease if it's producing a healthy fruit. There's a LOT of fruit tree diseases out there. You're further ahead, by my line of thinking, to give the tree supplementation to help it be healthier, rather than trust to luck that your next tree just magically won't have disease--as well as losing years of production while you're waiting for that tree to reach production.
To me your tree looks like it's doing good without having a lot of extra resources. I wouldn't mess with it much. If it was me, I'd give it some fish fertilizer and leave it alone otherwise, and enjoy years more fruit. And plant more young trees, because you can't have too many apples!
I think sometimes the best thing to do is wait, observe, and try to learn. What we think is the way things are supposed to be isn't always the only way things work for more natural farming / growing practices (and still often has good yields--like the "escaped" apple trees some people find that give abundant fruit in less than optimal growing conditions, with no pruning.) Sometimes we think we have to fix things that might not actually be a problem.
Have you watched this video about pruning?
I hope you find what works for you.