I agree that the way you do this needs to be right for you personally. I also recall Shirley Glass advising against getting sexual details until some safety and goodwill were restored, because those might aggravate the trauma. However, I didn't interpret that as details about other important matters and in any case, I felt free to ignore that advice because I knew that for me, speculation and imagination would be far, far worse.
Sadly, I have also seen several posters on here who have been traumatised more by not knowing the full story - there is even a current thread about this.
I also want you to think about the malleability of human memory. It is a fact that the more distance there is from events, the less accurate the recollection. The more guilt there is, also has an effect and recollections are sanitised or in your H's case, his own actions minimised.
From what you describe, this was a slow-burning affair borne out of deep friendship. There is much to tell and so much you need to know and understand.
Only you can decide what information you can cope with. People like me have a need to know everything, perhaps because I knew my mental health (and the relationship) would suffer if I was in the dark. In this situation, you are never on the same page as your H. He knows everything about what he did, but he won't yet understand all of it. You don't know everything and can understand less.
I wonder whether you have also told him the story of the affair from your perspective, because that might come as a shock to him and he needs to know it. Have you told him what was happening to you while this was going on? I always recommend this, because it is the one story that affair partners never realise - the effect of their behaviour on their primary partners.
As you might imagine, I wrote my story and when I gave it to my H to read, he was shocked to the core, but it gave him huge understanding that he otherwise might not have gained. People having affairs tend to minimise everything, including the effect this might be having on their partner.
Do what you think is right for you personally - and what you think you'll need, to prevent feeling worse later.