Developers come to me and say, hey, I don't want to get too deep in one tech stack, because a senior developer, it's someone that can get things done, regardless of the tech stack. Mm, wrong. Why? Just to get things done at a CN level, you need a deep level of expertise. The simple problems, writing a UI component, extending an API endpoint, I already assigned to the junior and mid-level devs in your team.
You will deal with unclear requirements, edge cases and nasty bugs. The only way to be effective at that level is by mastering your tech stack of choice. If you are a web developer, that's most likely going to be JavaScript. If you want to get to senior developer, you must first become a senior JavaScript developer. Technical depth always comes first.