I'm recently out of the other side of it and had a huge 3 stage decluttering in order to redecorate a room.
The reality is that it's very easy to gain too much stuff of little interest to children. And the more they have, the less they play with it. We gained a lot from gifts from extended family and hand-me-downs from a large number of cousins as mine are the youngest. Then there was the birthday parties... I could have cried after the birthday party when the entire class came bearing half the contents of the Toys R Us closing down sale.
Most of these things don't tend to be well-loved. We know the difference between favourites and passing novelty.
Break the mission down in to small blocks. You can achieve a lot with regular 15 min rounds.
I'd look at the playroom floor, set a workout timer into intervals of work/ rest, so I'd look for the most obvious theme... 2 mins picking up cars and putting away. In the 30s rest, I'd look for the next obvious theme e.g. 2 mins of duplo.
Use shoe boxes/ sweet tubs as temporary sorting storage.
I could handle bitty stuff like duplo that can be reassembled in any order, but there are so many shitty toys that need specific parts in a specific order that are just unmanagable. Tracey Island, a Playmobile fire station and a marble run were all memorably offensively expensive disintegrating shite that spent far, far more time in crumpled heaps and missing heaps in an unplayable state than being played with, and all got rage-binned... and they were not missed. The marble run was replaced with a basic Galt one and that got far more play because it was open ended and flexible than the fancy one.
Donate what you can, but the reality is that there is too much crap not fit to be sent on for future use. That's a design and society issue. There's not much point in blaming consumers who thought that they were buying a decent brand or people drowning in token gifts.
Life is so much tidier since The Great Sort Out, and sorting decent storage for the surviving toys, and we can now cope with tidying it; even the DS who at 3, mastered the art of hiding under the cushions in the book corner at tidy-up time at nursery 😂