4 yoe asking for career advice

Hi. I'm a web developer from Poland - 4 years of commercial experience. My first year of internship/junior position was fullstack dev - React, some Go and PHP and I've been working recent 3 years as solely frontend devloper with focus on React.

I'm thinking about slight career shift, as the decline of frontend positions and my previous job seeking experience (4 month of searching before setting new job) was pretty painful and depressing

I believe I've best chance to expand to fullstack role.. But should I actually do it as a guy working in frontend for the past 3 years? Do Udemy courses on Express.js or any other techs and just include learning experience in Resume? GitHub repos with small backend projects?

I honestly don't know, and I just want to secure a better place in market (especially Polish & rest of EU) for myself with broader skills. I also thought about trying Cybersecurity but I believe it would take me more time than going fullstack.

Do you have any advices or even hard recommendations for technologies I could learn? Any tricks that could be helpful in moving from frontend to fullstack?