Hey @recep!
Retool designed the Public Apps feature to support applications that are truly accessible to the public (examples could include an application that includes a form for collecting local news reports or a public-facing COVID tracking site). Using Public Apps with your auth to reduce your user count actually goes against the terms of service.
If pricing is a blocker for you at the moment you can reach out to antony@retool.com. He'd be eager to get you spun up, and see if a product like Retool Portals could work for you!
If you still want your own auth service, you could still use it to make sure users are authorized to take certain actions against backend endpoints by setting up custom API authentication in Resource settings.