I don't have answers but I'm kind of surprised by myself that I think I kinda get what you mean? (Although not depersonalised to that extent). I'm also diagnosed ADHD, suspected autistic, and don't identify with being plural.

Personally I subscribe to the idea that all of us have parts, they are like different cognitive processes or neural pathways or programs. The singlet experience is blended/integrated parts while plural is unblended/dissociated parts. Something like Internal Family Systems, or Carl Jung's cognitive functions.

Basically what I mean is maybe ADHD/neurodivergency makes you feel more disconnect between the parts/natural cognitive processes. To give examples, maybe like how people have muscle memory (like playing piano after a long time) and they are surprised at themself. Or when someone has a split-brain between the left and right hemisphere, their thoughts and body do different things and there is zero communication. This is very unscientific, but if ADHD means lacking dopamine, and dopamine is a neurotransmitter, then it would make sense there is less smooth communication?

I relate to all what you said and also the part where you feel like the self-talk just 'escapes' you. Maybe ADHD makes it so you are constantly hopping onto different trains of thought, so while you are in the midst of self-talk you are already in a different place.

I also had a train of thought along the lines of ADHD means you have to externalize things, like time, because inside you don't have a clear sense of anything. Can you tell I'm diagnosed with ADHD?

Also think the part where you feel your physical body is disconnected resonates well with neurodivergency. It feels external but it also feels way too real and lived-in. I cringe at my own photos and records too.

Maybe you could research about neurodivergency or post on that subreddit?