Are the readable replicas supposed to be long-lived (as in, I don't know, hours)? Or does consul happily converge even with ephemeral instances coming and going every few minutes (thinking of something like Cloud Run and the like, not sure if Fly works the same way)? And do they need to make a copy of the entire DB when they "boot" or do they stream pages in on demand?
Right now, yes, they should be relatively long-lived (e.g. hours). Each node keeps a full copy of the database so it's recommended to use persistent volumes with it so it doesn't need to re-snapshot on boot.
We do have plans to make this work with very short-lived requests for platforms like Lambda or Vercel. That'll use transactionally-aware paging on-demand. You could even run it in a browser, although I'm not sure why you would want to. :)