Welcome to FP world!
@ladco as a full time Java developer and a beginner in FP. This is what I would suggest:
With the gears shifting in Java, as you already know, you can always write Java in Functional way. This is how I started in FP, before even discovering elixir/erlang. I do program almost regularly in elixir, for practicing as a hobby.
When you are starting something of your own, you quickly need an MVP or might try out an interesting idea. The point is your TAT could be in the matter of hours or days, and this could be a matter of getting a new customer or packing up!
So, this is at-most important: YOU NEED TO BE PRODUCTIVE! you don’t have time to debug some unknown exception that popped up in production and have clueless about it. I think if you are still exploring, I would still advice you to look around, not just pick the first thing you see. Phoenix is a great framework, I’d love for what I can do, but for designing just REST apis, it just might a bit bloated (may be I wrong?)
anyways, here’s the bottom line:
You use Java to design your apis (It’s verbose? yes – but it just works. if bothered too much, then switch to scala/play) and put it production. then, on the side, you can test the same with phoenix on the side – in dev. With startup, I believe your budget will also be limited.
Good luck!
Chandra
PS: you can always ping me if you need any help with Java 
