Canonical loop ate a client's whole product index

Canonical loop ate a client's whole product index
Filter pages canonicaled to the clean category. Sounds right. But the category page paginated, and page 2 canonicaled to page 1, and a filter ON page 2 canonicaled back to category page 1.
Google saw conflicting canonicals and just... ignored all of them. Picked URLs itself. Wrong ones.
The move:
— Filter/sort URLs: canonical to the clean filtered version if it's valuable, otherwise robots.txt block them outright.
— Pagination: self-canonical, never to page 1.
— Never chain canonicals through a page that itself canonicals elsewhere. Canonical to the final destination directly.
— Audited with a crawler showing the canonical target column. Sorted for any URL whose canonical target ALSO had a canonical pointing away.
Watch sort-order params. ?sort=price is the sneakiest canonical-loop source nobody checks.