If we didn't do the whole class party ds4 would pretty rapidly disappear from the party circuit, it's petty but also how it works here. And actually the boys love them (or rather did, ds3 attends an autism school and parties are a whole other species there, we tend to go away as nobody likes them and it's all just done through a sense of duty, and ds1 and ds2 are old enough now to choose a family meal).
But yes; clear outs are an obvious solution, as is rotation. With 4 we still have plenty as they have different ages and levels (SN is a factor here) but it is largely managed by the God of Trofast and the fact once they get to 10 or so thhey prefer hobby related gifts- such as books, beads and art materials. We are drowning in books but that's a positive.
Anyway we're happy; we have to prove to the tip that we are not fly tipping as we use it so much but as long as we keep on top of the storage space we have allocated (a lot) so be it. My family were never going to do the wooden toy thing, and I certainly would never be rude enough to tell them what to buy so it was one battle I never chose to take on.
Detox- so much! social skills, turn taking, mimicry ( a key developmental skill), the very important skill of replacinig a toy's intended use with something else (and any toy can be used for that, I remember as a child turning a plastic push along car upside down and using as a pretend spinning wheel! Clearly I am pretty old).. Shapes, construction, the plain old concepts of mine and I like this so must care for it. What noises they make, how the bounce off walls (!), whether they float or sink, does thee cat fancy it, why won't it work- is it a button, does it need batteries?, what other games can I play with this. How do I negotiate with this toy to get THAT toy from my brothers.
Kid are scientists; and they will be whether they have a box or a cupboard full of plastic toys. The nature and amount of toys is unimportant as long as they're not bought on credit (!), available, safe and allowed to be used as the child wants and not just as the parent intends.