I also had about 2/3 years of experience when I switched from Rails. It did take me a little while to become productive with elixir and phoenix. But now, I think I'm way more productive in elixir than I ever was in ruby. For new projects I always prefer to use elixir when possible. I'm more productive in it and I enjoy it more. The one big knock against elixir and phoenix used to be the ecosystem, but I don't think that's much of an issue anymore. You can find a package for most things now.
As for the lack of stackoverflow answers I wouldn't worry about that too much, I've found the elixir community on this forum, on slack and in IRC to be very helpful.
I also think learning elixir made be a better developer in general, because learning functional programming forced me to think about problems in a different way. Even now when I go back to Ruby I tend to write in a more functional style.
Finally as a startup if you do need to hire in the future it will probably be much easier to attract good elixir devs than good Ruby, Python or PHP devs. I know this is true in my area (Toronto), and I assume that its true in others as well.