Until very recently I was a size 24/26 and flew often. I never encroached on anyone else's seat and I'm sure you won't either. The seats are wide enough to accommodate my hips, anyway!
I only ever had to use a seat belt extender once (thanks, EasyJet 🙄). I found it was usually best to lengthen the standard belt as far as it goes before sitting down, then fasten it across your hips, not your belly. This gives you more wiggle room. Usually it did up with a little bit to spare, but don't be embarrassed if you need an extender. No one else cares, honestly.
I usually book an aisle seat so I don't have to shuffle past anyone to get to the loo, but I can see the window seat would work too so long as you stay put.
If you have a problem with the tray table, just pop the meal on your knee and eat the hot part first.
Everyone saying you need two seats is entirely wrong. Only the most extremely fat would ever need to do that.
And don't be embarrassed - I'd much rather sit next to a polite, considerate fat person than someone smaller but objectionable! I've sat next to people who stink, who are aggressive to cabin crew, who are having loud arguments with their family, or who will not shut up! All those are much much worse!