I recently found a way to get my 4 year old invested in parting with things he has outgrown. I sell them on FB or at the children’s resale shop and he gets the money to buy new sets for his lego collection. Once he realized that old toys he doesn’t play with anymore could turn into new toys he loves he went from “I still like that” to “can we sell this?”. He will literally look around the house for toys to part with now. I used to discreetly toss anything broken or incomplete but I have started involving him in that when I can so I’m not just throwing his stuff away in secret and he gets some experience parting with worn out items. I also give him a small amount of lego credit for items we’re donating to the thrift store. The key for us was to help him focus on making space and freeing up resources for his current passion.