Thank you! That's great and it is possible but... With some limitations.

The idea is from sign up form to a D1 Database that can be accessed from the worker itself.

That's not possible without updating worker bindings like you showed and further - there is an upper limit of 5000 bindings per worker and just 5000 users then becomes the upper limit although D1 allows 50,000 databases easily with further possible by requesting a limit increase.

edit: Missed opening.