Ask HN: What's the easiest way to setup a scalable API developer platform?

Hey HN, Let's say you just finished building the core functionality of a highly commercializable API and now you want to release it to the world. Congrats!

How would you go about setting up a portal where people can: 1. Register as users of your API and later login. 2. Modify their account information (email, password, name and so on). 3. Set up automatic billing, either via a prepaid wallet (similar to what OpenAI uses) or via postpaid charges. 4. Track their API usage.

I've looked into lots of options but I'm struggling to find one that is going to be easy to setup while also being scalable.

There are many API gateways that offer portals but suppose that you don't want to use an API gateway because they often charge consumption and/or transaction fees, not to mention the risk of lock-in if you're unable to easily port your users.

I've even been looking into Wordpress because it's definitely scalable and easily portable, but it can also be a bit clunky and this use case may require a lot of upfront work and quite expensive plugins that are not guaranteed to be easily adaptable. If there's plugins that people have managed to get work for this kind of thing though, I'd love to hear about them.

There's SaaS boilerplates but none have really impressed me apart from SaaS Pegasus, and it seems like some work would be required to adapt them to metered billing.

Short of rolling your own portal, how would you go about this?