It would be great if it was possible to edit the SEO main category of products in an easy efficient way - as in, not having to do this product after product by hand or via API because the former takes forever if you have a lot of products and the latter exceeds what most shop owners are capable of doing on a technical level.

A DB mapping for it in Import/Export would be a huge improvement already or perhaps a way to do this via Bulk Edit in the Admin would be another way to handle this. That way, you could go into the category overview, select a bunch of items and then set the main category for them in bulk, saving you lots of time.