What should I learn today to have a strong career for the next X years?

I’d ask the question of what you already know.

If you are a self-taught developer learning about compiler techniques is a major powerup. So many people are obsessed with ‘functional programming’ and go looking for the rainbow’s end for years and… no pot of gold. If you understand compilers it is like going from 1-dimensional chess to infinite dimensional chess.

Java, SQL.