There is no programming job in the world that would take me
six months full-time to become effective.
Maybe a month, if it's an awful setup in an awful, unfamiliar programming language with unhelpful colleagues. It sounds like you're assuming if you hired a React programmer to work on your React app, it would take that programmer 1 month to get up to speed, while it would take a Haskell programmer 6 months. I claim: if your codebase is complex enough and your situation unusual enough that it would take a React programmer 1 month to write productive React code, the Haskell programmer will take 2 weeks. This is because the vast majority of the learning curve is in your domain and setup, not in how React components work. (Assuming for sake of argument they are actually smarter.)
People vastly overestimate how long it takes a decent programmer to learn new tech.