If anyone else is thinking of switching from Rails to Phoenix then I humbly recommend my course Phoenix on Rails (http://phoenixonrails.com/). Sorry for the self-promo, but if you use the code HIT8RUN (valid for the next 48 hours) then you can have a 10% discount.
[0] https://solnic.codes/2016/05/22/my-time-with-rails-is-up/
Phoenix has undergone some bigger changes with recent versions due to the Liveview development, which were a bit annoying to keep up with but in general, everything is very stable and well documented.
Elixir in general is one of my most beloved ecosystems in terms of develper productivity. - Opinionated, configurable formatter, type annotations and checking, very good test and doc tools, awesome community...
It's everything I know from the Ruby + Rails community years ago but dialled up a bit. I haven't done much Ruby work in the last 5 years so I can't provide a direct comparison but I always felt like you really feel the extra 10% of polish Elixir has over Ruby due to the core teams strong involvement in tooling and developer experience.
The only even more streamlined experience for in recent times was Flutter (as long as you don't stray too deep into ffi/native code)
I always just look at the number of bugs phoenix has compared to comparable frameworks and it makes me very glad to be using something this stable.
It's my goto for any hobby projects these days (used to be Django).