This may or may not help you with your specific choices but I just posted a blog on hosting PWAs with Phoenix. I use our Websockets client as an example. This particular one is pretty simple and only uses a dash of AlpineJS but no reason you couldn’t go all out with Vue or similar.
For what it’s worth, if you plan to integrate with Phoenix routing and hosting (not just APIs), Vue is likely to be simpler than Ionic. Ionic requires you to use their build tools and I have found fiddling there to be quite tricky. With Vue, you could get started by simply dropping in a script tag and binding components to elements on the page. You could then do what I do in the blog and just point the router and assets the right way.
