Productive stack for a solo developer

I'm looking for a productive, minimal headache inducing stack for a solo developer. It will involve a web app, and quite possibly a mobile app down the line. It will involve a lot of interactive content (delivering interactive educational material), but minimal technical complexity, and no scaling issues (very niche topic - never going to have a lot of requests per second).

Obviously makes sense to stick to a language I'm comfortable in - Ruby, Python, C#, Elixir, PHP, JS. Any thoughts/advice on productive solo development?