[pawelduda]

If you want to make the most out of it - probably continue with JS for the backend - throw in TypeScript. You'll also be able to use that knowledge in frontend (and anywhere else really) as a bonus, and it's not going away. IMO your best bet.

Ruby is my favourite but at the same a niche language compared to JS or PHP - I'd check the local market first if you're under pressure to land a job - there are many great offers with competitive salary if you're open for remote work though. Note that by diving into Ruby means a 99% chance of working with Ruby on Rails - it pretty much dominated the ecosystem.

PHP is very popular still, but I don't think it's a solid choice career-wise. A lot of ways to shoot yourself in the foot, the legacy code to deal with, and oversaturated market. Still have PTSD from that.