How about you try first to rewrite the frontend without all this junk, vast majority of this stuff is already available in html5 and css, and if you hit limits, then try something minimal like https://alpinejs.dev/
Of course you can also try to write the frontend with LiveView. It sounds like you want to try it and are looking for excuse So I do encourage you to try it It is additional complexity initially, but it might end up being better solution than the above, that really depends on specific details of the websites in question.
@jchatard I’ve been through something completely similar. Have an e-commerce website that started on a PHP stack and had to change the front end for performance reason.
It went through multiple iterations:
Normal PHP + Riotjs ( Vue like JS framework )
PHP on the backend + Elixir for the frontend + Riotjs
PHP + Phoenix and Liveview
I basically rebuilt the whole model layer of the PHP app on Elixir and I plug directly to the DB of the ecommerce platform. Cart operations are handled in liveview and then the elixir layer “simulate” the user session and communicate with the ajax api of the ecommerce layer.
Only things still served directly by PHP to the end users is the payment page.
Results: Between 95 to 100 pagespeed score on desktop and mobile. My rankings in terms of SEO have increased 4 fold.
I’m actually in a different zone, I’m keeping the back-end in PHP, and rebuilding the front-end using SolidJS ! It’s still early in the process, but I’m at 98 in Lighthouse as of writing. Which I’m super happy about.
I would vouch for you to do it, because in the long run will be a wise decision.
Deployment
Its not that hard now, but it was a little more complex in the past, but some of the complexity its just people not being used to deploy an application built with a compiled language, at least it was for me (PHP Developer before).
You need to take your time to understand the differences between a compiled language and a dynamic compiled language:
Compiled time configuration vs bootime and runtime configuration.
Code that you write in some parts are only exercised at compile time, not at runtime. This can bite you when trying to configure the application with runtime configuration.
More exist, and others may contribute to the list.
After you understand this bit deployment will not bite you on configuration, thus you just can use mix release to build one and deploy. You can learn more with the book from @miguelcoba:
You can also use the built-in Phoenix command to deploy to fly.io and have your app running distributed across the globe.
Elixir
Elixir is a compiled language, but most of all is a functional programming language and your brain needs to make the switch on how to think about coding in a functional way, and the only course that clicked for me was this one:
Don’t dive into coding on it without understanding the programming model and how things work, and the free chapters on this course do an excellent job on it:
You can even go and subscribe the paid version to master LiveView.
Performance and LiveView websites
I am always amazed how developers forget on very important bit in the performance equation of a website, the hardware and internet connection of the end user. It’s fast for you on localhost and even in production, but the chances of you using the latest hardware and a fast internet connection are great, plus you may be very near of your servers location (speed of light is slow for users in the other side of the world). I invite you to read this discussion:
By the end of it some websites using live view are shared.
So I do encourage you to try it 
