You've probably started school already but I just wanted to weigh in. I'm in law school, with CFS/fibro, so not exactly grad school but many things are likely similar. Hopefully you've connected with your university's disability office (e.g., Disability Resources for Students) to set up accommodations. (You may know all this, but for the benefit of others on the internet...) Even when in other contexts CFS or other chronic conditions aren't considered "disabilities," universities are able to take a pretty broad view of this when setting up accommodations, as they can do for students with things like generalized anxiety or learning differences. My disability services coordinator was able to suggest a bunch of accommodations that I hadn't even thought of.
Some of the simplest things have been quite helpful to me in starting conversations with my professors, and also saved me from having to disclose the details of my medical condition because everything I might need is laid out in this list of accommodations. "Access to food, drink, or medication" is a helpful one, particularly to clarify whether its ok to eat in class or if you should step out. "Ability to take breaks" is also useful, because if I'm in some pain or just want to stand up and walk around for a minute so that I can focus better in class, I'm able to head outside - and even if I do this once or twice every class the prof knows it's not just because I'm bad at planning my bathroom breaks or something.
Copies of materials are very useful, if I need to review things because focusing in class was hard or if I actually miss class. Many profs do this already but there's extra insurance when the disability office "requires" it because its an accommodation: audio recordings of lectures, copies of powerpoints, and copies of other displayed materials (e.g., a photo of what was on the blackboard) are all on my list.
Like the previous poster said, if you know you will need to miss class frequently you can take some extra steps to ensure this is approved by the university. I believe there is a formal accommodation that covers missing class but the policy for how you follow-up on that will probably vary by program. There is also an accommodation for "reduced course load" so that if you decide to take fewer classes at a time in the first place, you can remain eligible for federal financial aid even if you're under the credit load usually required for that. That's something that could
open up more space in your schedule for appointments, though it would probably mean taking an extra semester or more to finish your degree.
Because of fatigue/brain fog issues, I was able (quite easily) to ask for testing accommodations (1.5x time and reduced-distraction environment). I also have 1.5x time on "quick-turnaround assignments" (mainly if these are graded), such as writing assignments or take-home tests that you get 2-3 days for, so that if I feel sick/fatigued or have other issues there's some more wiggle-room to get those things done.
If you're doing research, you may want to talk with your advisor or the program directly about
what accommodations or flexibility you can have in that. Even though you're never required to disclose the reason you have an accommodation, if you're working extensively with a particular prof, who you feel you can trust, they may be able to help you better if they understand a little more about
Best of luck! Grad school is totally doable even if you find you have to do it in a different way from "everybody else." Plenty of pathways to success and fulfillment