Interactive auth sounds attractive at first but it's really the wrong place for an answer once you look at all of the ways captive portals are used (i.e. more than just "check this agreement box"). You really need the power of the browser to display a custom form behind the solution or you end up with n+1 solutions instead of replacing captive portals.

Something like a DHCP option or NDP option ends up being a lot more natural: "Hey, here's your IP along with the information needed to access the network" is already a function of that layer. Some devices (e.g. macOS/iOS/iPadOS, Windows, Android) take a similar approach in the reverse by probing for a specific test url. That's also a bit hacky and unreliable (e.g. it can falsely trigger) but some minor standardization of it to e.g. a well known DNS name could be another good option.

There already was! It was called Passpoint R2 Online Sign-Up, but it never got traction on the phone side of things, so it's now being deprecated by the Wi-Fi alliance.

It's really a business problem. IMO you shouldn't have to solve this just because you've gone indoors – you already pay a carrier for connectivity – but many carriers don't want to own that responsibility.

CGNAT is ok for IPV4 because it provides us some level of protection against state ( sponsored ) actors.

Is there even an alternative to captive portals?