And it's not just Elasticsearch. That's just one example. I would come up against many other requirements. Here's just one more: https://nats.io/download
Edit: And you're right, Haskell is something I should probably look into more. I haven't thought about it too much since I already do a lot of functional programming in JavaScript and I don't hear too much about Haskell being great for creating API servers.
One of my other concerns is that the project I'm planning will be a long-haul, and will have several other developers join in future. I want to have a decent market to choose from. And, being in Australia, it's small enough as it is. I'll shy away from remote workers since it's relating to sensitive healthcare data.