I would like to focus on the api or web-app rather than the sign up process. Is there a minimal User Sign-Up service or portal/tool like thing that wraps access to an api or web-app that you'd recommend? Is this possibly a bad idea?
I'm guessing even the initial integration could be a bear since most things Auth tend to be. Open to ideas though.
You haven’t mentioned what stack you expect the solution to be.
I use my own boilerplate (https://usegravity.app).
Services are a good option depending on your needs. Just be aware that you may run into a wall at some point in terms of customisation and it can be a nightmare to migrate down the road.
If your needs are fairly standard and you need something running quickly it's a good option, otherwise use something highly customisable or roll it yourself :)
You can take a look at Rocket (https://rocketapp.me/)
Do you know Auth0 and Firebase Auth?