It's good to play around with different frameworks from time to time, even if it's just to form an initial opinion on. I've been programming for 15+ years and the only constant is learning new things.

From a career perspective using it enough to know whether you'd like to or be willing to work with it in the future is probably enough. Then when you're looking you know whether you want to apply for jobs focused on it.

On that topic I've been on the market and haven't seen Svelte mentioned a single time when searching, granted I've probably only looked at a couple hundred listings (most being WFH).

From a career perspective, think of languages and frameworks as tools. Knowing how to work with more tools broadens your horizon about what you can achieve and how efficiently. Sure, you can specialize on certain tools, but these come and go.