I suggest you examine what you’re looking for from your startup. Is it to learn new languages & programming paradigms and to have fun, or to validate a business idea?

If it’s the former, great. Keep going with Elixir, ask questions when you get stuck and have fun. If the latter, I suspect using a language you’re not proficient with is a mistake. You’re validating an idea, odds are you’re going to throw away or rewrite the code you’re currently writing. Use the language that you’re most productive in, so you can get to that stage sooner.

I will echo what @lukerandall said. “Startup” implies that you have money - perhaps your livelihood, on the line. In that case go with what you know.

Now to your point of retooling your thinking. Making the move from the Object Oriented paradigm to the functional paradigm is difficult. For the folks I know who have worked through it, there is not really any special trick. It takes practice, plain and simple.

What worked well for me was working my way through a lot of small problems - using something like exercism (http://exercism.io). With the way functions compose it seemed to work well for me to try little things and then build them together into bigger constructs. I also spent time using Functional Paradigms in applications that were otherwise Object Oriented (Swift and Objective-C in my case). After a while you get a good feel for when a problem is particularly suited to the Functional Paradigm which helps.

Definitely avoid transitioning from OO programmer to functional programmer at the same time as you transition from engineer to entrepreneur. Both are quite challenging endeavours, which you’re better off tackling one after the other.