If I wanted to learn more back end I would just learn more about PostgreSQL features. It seems like a solid long term investment. You can bet on it even if the Rails vs. Node vs… thing is uncertain.
There is a lot of innovation running atop of it too: Suprabase and things like that for example (which are more funky things that can be learnt as needed)
I second this. I've used PostgreSQL for a long time, but in the past couple of years I've spent time learning how to use TimeScaleDB, PostGIS, and the JSON data types. Recently I've been working on a side project that uses Supabase (built on PostgreSQL). There was a post on HN yesterday on PostgreSQL full text search and now I want to incorporate that as well.
There is a lot of innovation running atop of it too: Suprabase and things like that for example (which are more funky things that can be learnt as needed)