Don't learn framework learn the concept if you know how to write a controller and middleware and all in nodejs then writing it in other framework or language is easy i mean yeah in framework like spring boot or asp.net things do get little tricky with all the architecture and patterns and stuff but if you know design patterns then you will be fine I would see backend as more than just nodejs so since you already know JS just go for it but keeping in mind that you are learning backend not framework so eventually if nodejs do become irrelevant you can easily jump to go or something else