Milly, I also have a hard time with my son.
I hope I'm not adding to your problems by suggesting this, but have you considered ASD with daughter?
Her issues sound awfully like my son, who's part-way through formal diagnosis at this point (and which may mean that I am too ready to see the same issue in others even if it is not actually warranted IYKWIM).
In the meantime, I've found getting rid of his stuff (such as stones, beads, random bits of paper and so on, worked best by putting each type of item in a dedicated container. Like one box that printing paper came in for paper items, a decorative box for stones, and so on. The container stays in a cupboard so that it is not too salient in his mind. And when it gets too full, items very quietly disappear from the bottom of the container. Nothing is ever overtly thrown out in front of him, and it may take several months between it coming in and going out, but at least it ends up OUT of the home.
I actually feel bad for doing this, but don't know how else to do it...
For self soothing, I give him things to fiddle with. The most recent one was an Indian key ring thingy which has a mirror and tiny little bells, and therefore makes very pleasing (to him), reflections and sounds when fiddled with. And I've found an upfront discussion with hime about how some things WILL break or WILL be finished with or WILL be gone after their appointed time is (only sometimes) helpful.
Back on Kondoing - DH read the book himself! I thought he'd sneer, but he was actually impressed by her insights, and has removed bags and bags of books and papers over the last few days!