To be short and sweet: You should focus on what's best for your clients. No client will ever say to you "Oh that react app is much cooler than that Vue.js app, thank you for choosing that!" simply because all they want is a working product.
You should focus on what you do best for your clients, and on the side learn stuff to stay up-to-date in your field. With focus comes quality, otherwise you might risk ending up as a jack of all trades, master of nothing.
After 13 years I still know nothing about 'backend stuff', simply because it's not my thing. I can't get my head around a huge code base and are much stronger on the visual side of things so that's my jam and I utilize the skills of other people to create the things envisioned by my clients.