Magento – After Data Migration Product Listing Not Showing

Only orders and customers will be migrated using delta migration. DevDocs is now updated to explicitly state this

https://devdocs.magento.com/guides/v2.3/migration/migration-migrate-delta.html

You will need to manually export products and categories and import them to your new store.

In the end I

1. removed the "Default" attribute sets from eav_attribute_set,
2. set all categories to attribute_set_id 3 in catalog_category_entity and
3. set the default category attribute to set to be 3 in eav_entity_type where it was set to 20 (and also the default attribute set for products to make sure they were added like the imported ones).

This corrected the problems with adding new categories in the admin and showed the products in the front end categories.

I had to fully clear all the caches and reindex for this to work.