I mainly write in js...is ASP.NET worth learning for further career opportunities?

If you want to become fullstack, you'll have to learn backend too. Multiple.

Lots of jobs in ASP.NET. So worth having a go.

It is a good idea to learn multiple languages, as the one you use most often might not be well suited for a particular problem 😉

@DLMousey I'd say the complete opposite where I am, for some reason. I've not seen many PHP jobs at all

In my region, .Net is primary. In my country, it's not. This is why you diversify, learn multiple. It'll take time, but it will boost you to senior, and make you very attractive in interviews. Today, if you are not full stack, you'll be hard pressed. Full stack is a term needing more definition, imo.