yes, _add_url() is not public, but the implementation shows that it’s just a wrapper on app.server.add_url_rule()
we have discussed the restructure of dash code internally, but the easy-switch-to-another-framework won’t be available in the short term.
another thing to point out is fast-api requires 3.6+, and now dash has to support both python 2 and 3.
again, the porting to fast-api is feasible in python3, but I’m afraid there is no easy switch right now.