Hello!
I was in band during middle school, and I plan to get back into music while I'm in uni! I'm glad that you posted this question here, because I'm wondering the same thing!
I learned my songs mostly through ear; I learned the melodies and memorized them in my head. That may or may not work for everyone though.
That's all I can really advise, unless you can somehow position the music stand closer to your eyes. I hope this helps, and good luck!

I can't really know what would work for you since it sounds like your visual issues are very different. I used to just enlarge sheet music and got by just fine. Now I struggle to see it at all on the stand. If I enlarge it it has to be so large that it's hard to use effectively because I can see so little at a time. I miss small markings and my eye can't track everything I need to at once (my eye muscles are sluggish as it is). I learned the braille music code and that's not any more convenient. It's very bulky, especially for complex piano music, and presents the same problem of having to memorize everything. I'm not great at reading braille music, and I can still read print well enough close to my eye that if I want to memorize I wouldn't go that route anyway. In haven't yet found a great solution, but what I generally do now is use a large screen to display music and a page turner pedal similar to the set up with Limelighter from Dancing Dots (you should look them up and read about it). I've also thought about trying a monocular device to clip onto my glasses, but again the field is generally so small. I'm getting by, but none of these really solves the problem of only seeing a very little at a time. It might work for other music that only consists of one line, but piano music is much too complex for this to be really effective.

The one thing I can definitely recommend is that you try a page turner pedal if you haven't already. That's a huge help when you have music that's really magnified and only displays a small amount at a time. You could also look into modified stave notation, as you mentioned some sheet music works fine for you and some doesn't. It can make a big difference.

I am a musician by trade and I can't read music at all, which is a bit of a limitation but it's not a disaster. People will say learn braille and then learn braille music, but honestly for the sort of work I do it's not all that practical because it's often at very short notice and I would simply lose a lot of work because they're not just automatically going to have it in braille. What I mainly do is have a buddy play through it on piano so I can learn it by ear, but then I'm mainly playing bass and a bit of lead guitar so practical to do that. If you were trying to learn some enormous piano concerto it would probably be a bit tricky but as you say you can only play one note at once on a trombone! After a while I found we figured out ways to do it that work where they pick out any parts that aren't obvious in more detail and talk about any relevant notation. I have played with symphony orchestras and felt terribly out of my depth for all sorts of reasons but not usually because I couldn't read music.

It means relying on other people a lot, which I do not like on principle, and I end up owing a lot of people lunch, but if it's that or not do it I'll put up with it.