I've been doing web-development on and off for 20+ years and know PHP quite in and out, but I see that it really lacks compared to other more "native" languages tied into Shopify. My goal is not to build huge applications for third party, but rather develop our own shops with custom functions.
If I should choose to learn myself something else (or "transition") into a new language - would Ruby be the "only" logical choice? I also see that Python has a quite good library on GitHub with support for pagination etc so it would basically cover my needs.
In the end it will come down to your own preferences, but I suggest you play around building something using different languages to see which one you like the most.