For the backend: it’s usually hard to find Drupal developers, so that’s a good one. Drupal has a huge front-end side too, so you get that with it.
But that’s what I would pick currently as possibly high in demand. Just look at the job listings.
I think it’s not true your skills will transfer quickly to another language / toolset. I’m highly experienced, and have used Elm quite a bit for over 2 years now, but I definitely cannot yet program in Elm like I could in other languages. I.e. I have no full mastery. There’s a lot to a toolset, and a lot of culture that one needs to learn, and you learn that by doing only, i.e. slowly.