Collecting specialities is important, but never think of yourself as a "front end engineer" or a "mobile developer", you're a software engineer that has happened to specialize in two previous fields.

My last three jobs I've done big data, SRE, network servers/socket programming, distributed systems, and more.

Some companies will hire you for your speciality, and others will hire you because you're good at learning a speciality, and they might have a new one they want to point you at.