This sounds like and A/B problem. You’re asking how to do A but what you really want is B. But you’re asking about A because that’s how you think you’d get B in the ipv4 world.

Your ISP should give your router a /56 prefix via PD, which you can divvy up into 256 networks internally. Alternatively, if you don’t need globally-routable addresses, just pick a ULA prefix and have your router advertise it internally.