@antimony There is certainly so much grief for the life your partner would be living without his illness. (Still grieving mine.)
Reading through your post, the biggest issue seems to be how to make the most of this appointment with the new GP. It is sensible to go to the new GP with a clear list and timeline of symptoms and the way life has changed. To get a useful diagnosis, the doctor needs the whole picture.
It is your partner's journey, but you are an important part of his life and, next to him, presumably the most affected by his illness.
I think my suggestion would be that you write out that timeline and list of symptoms as you see them. Especially write out clear, concrete examples.
I would include his ambitions and abilities, and how they have been affected by his chronic fatigue.
I would include symptoms from the CFS list that are relevant, eg if he has unrefreshing sleep and post exertion malaise.
You can avoid saying chronic illness and life long disability, just document the reality that you have seen. Be clear about the 6 years and the patterns you see.
Then, respecting that it is your partner's issue, I would give the list to him and ask him to consider if he thinks it represents the reality of his health issues, if it would be a good list to take to the doctor. And I would abide by his response (I hope).
A proper diagnosis is not life crushing, but a chance to make the most of that little life, and stop it from getting any smaller. Good management might even make life quite a bit bigger again.
You will sense, reading this, if it is something that may help. Ignore otherwise!
But best wishes