I am really upset because I am not able to take AP Biology this year because of scheduling conflicts. Is it really hard to self-study? Would AP Environmental Science (my school does not offer this) be a better option to self-study? What resources (websites, textbooks, exam review books) are the best for both of these?
AP Enviro is fabled to be one of the easiest APs, while Bio is supposed to be one of the hardest. I would say do Enviro as your self study.
I don’t know exactly what you should do for resources, but for a start, look for the in class textbook (probably on amazon) and the AP review book. As for the review book, there was a thread a while back that listed what books were the best for different AP classes, so maybe look for that.
The hardest ones are like music theory and the languages 
But from the ones you said, I agree with Otter. APES is extremely easy and AP Bio is extremely difficult. That said, it’s not impossible to do well with AP Bio but you’ll need more time. I know somebody who got a 5 on APES with less than a week of self study.
I fortunately ended up being able to take AP Bio after all. I still might self-study APES if I find that I have the extra time to do so.
Don’t forget to include your academic strengths and interests. Only you know which classes are easier and which are more difficult.
Cliffnotes (at least used to be, pre exam revision) the best study book and that + intuition could earn you a 5 on the exam. The internet had a copy if you google lol
AP Lit/Lang I feel is comparable to studying for the SAT… With my strengths you can’t really master with your intuition. It’s different than any social science, science, math, and even foreign language courses where you can’t just learn it by reading a book. And the type of MC questions they ask and analysis essays you might completely avoid depending on your professor for English in college. The only relevant item is the argumentative paper.