Would it be possible to have a API for another database with its own topics, posts, search and integrate that into Discourse so I can use Discourse as a front end and that as an backend?

No. Think about import your data.

לשלב בסיס נתונים קיים ל-Discourse

what (blox) 17 ביולי,‏ 2023,‏ 10:03pm 4

Is it possible to import 500gb+? Or is there a size limit