You should take a closer look at AppWrite, AzureB2C, or Google's Firebase. You can even have Azure or Google connect to your own custom identity provider. It'

You should take a closer look at AppWrite, AzureB2C, or Google's Firebase. You can even have Azure or Google connect to your own custom identity provider. It's only a few lines of code to implement user management using identities you control. I get not wanting to get locked into these things, but once you learn how they function, you can switch between identity providers in minutes. I do get the sentiment though, but for something like this my vote would be for certificates instead of basic auth.