Great strategy, I love how elegant it is, but I do wonder about two things regarding this method:
Testing it (especially with async integration/e2e tests, which I prefer) would be tricky, I'd want some tests to verify that the "GenServer write cache" is more or less counting correctly under some (potentially stressful) scenarios that I can stress out by having other things run in the VM.
From what's presented, I'm not sure yet how to get this to work with distribution, in the case that you would want to drop this into a distributed phoenix setting. Do you start the pages :global instead of registered? Do you start a Registry with :global, and have the cluster restart it if vm that hosts it dies?
I am not an expert at distributed systems with Elixir/Phoenix, but would Mnesia (https://en.m.wikipedia.org/wiki/Mnesia) solve that problem?