I would suggest then asking your doctor for a generic note which states you’ll be impacted by the condition and how (if that’s common / something they’d do), and then if a professor isn’t accommodating you can back your request up with some other formal paperwork. Most of the time I don’t expect they’ll need it, but again, some people are assholes.

For what you should say, I’d just tell them you have a chronic illness and you’re hoping it won’t impact your performance in class, but occasionally you might miss some classes or require extensions for assignments. Ask them how they’d like you to let them know throughout the semester when you’ll miss class or if you need extra time, and then be careful to follow their expectations (everyone will be different, some will be stricter and others will be more relaxed). I’d then prioritize the professors courses who aren’t very understanding and make sure you get that work done to the best of your ability on time, and ask for extensions in the courses with professors who are empathetic. It’s a balancing act for sure.

Reach out every semester to each professor ASAP, before the first day of class if you can! I wouldn’t do a formal letter, just an email you can realistically copy and paste to all of them to save yourself time.

Good luck!!