So, I went for the promotion and got it.
I think key to my situation has been transparency from when I was initially diagnosed: with HR, with my boss, with his boss, and with some of my team. I have been pleasantly surprised how supportive most people have been. Raising awareness of my symptoms, including the fatigue, has generally only been a positive thing.
I currently work from home one day per week and I've set expectations that at some point in the future I will need to apply for a flexible working arrangement (e.g. working a 4 day week). I'm going to try and be honest about what I can and cannot do at work, so for example I've already cut back on travel. I'm sure there will come a time when I have to bite the bullet and stop my current role altogether, but for now I'm continuing as best I can. Despite all this, it's still hard every day.....
I'm lucky and not every employer is as reasonable as mine but I would encourage other people recently diagnosed to not try and hide their condition, and If there is an occupational health facility available then it's a good idea to register early.