You don't write an API to port stuff to your competitor. You write a JSON or CSV export and competitors can then make an import tool for your data format (and vice versa).

Is this really an effort? It's basically a JOIN over a bunch of tables or maybe the JSON state tree of your SPA and that's about it.

Chances are, your startup works with all data of a user and has a way to request all data from the DB anyways.