@Onetwothree45 Being able to make decisions over big stuff including to get rid, generally is easier than dealing with huge numbers of small items, especially if those items are in good condition and eminently usable.
I have a large cupboard full up with with cleaning materials, from when I was still compulsively buying. Yes I could get rid, and buy more each week, but logically rather than spend more on replacing what I already have, I’m keeping them because I’ll work my way through them, and tbh having sufficient cleaning materials of all kinds, (clothes, human, and house) makes me feel secure.
(I blame my childhood, and your daughter could become me in a few years.)
Your partner is seeing absolutely everything he has, the way I see disposing of the accumulated cleaning materials.
He does sound overwhelmed, but he also doesn’t sound ready for what he needs to do. There is a journey from the point that you realize you have too much stuff, and try to do something about it, to discovering it's not as easy as it ought to be, and that you have an actual problem with acquiring and discarding.
Until you’ve realized the problem isn’t the stuff, it’s the symptom of what's wrong with you, moving forward is entirely random.
You are clearly also not ready, and not seeing that the problem isn’t how much stuff, what kind of stuff, if the stuff is organized or not. The problem is he has a disorder and the stuff is the symptom, not the actual issue. As long as you are only able to see his symptoms (the stuff) as being the problem, you will be confused and of little use in him addressing whats wrong.
My 'stuff' is clean and organized, much in storage. I'm pretty in control of acquiring, and am slowly discarding, but the underlying condition still has me firmly in it's grip, no matter how the 'stuff' is presented. The 'stuff' is the symptom that's forced me to stop ignoring the underlying problem.