There seems to be two API endpoints that are made to reorder collections.
REST Api - Updates the ordering type of products in a smart collection
and
GraphQL - collectionReorderProducts
The REST endpoint seems to be broken when it comes to reordering products - don’t waste your time with it. Ask it to move 20 products to the top of the collection, and you’re lucky if it moves anything.
I’m testing the GraphQL endpoint now - hopefully it will work.
Something has to work, because the app ‘Power Tools Suite’ is able to shuffle collections randomly, and I think the above endpoints are the only ways that they could achieve this.