According to my knowledge, most of the developers working on Bitcoin core are early miners who work voluntarily to develop the ecosystem out of their enthusiasm for Bitcoin, and this is why we see only people who are raw Bitcoin enthusiasts operating the Bitcoin node these days.
This is not true and one does not have anything to do with the other. Actually, a lot of the most active developers now are part of the new generation that can't be considered early anything.
Bitcoin software is mostly open-source, so it mostly relies on donations from third parties.
That means you'll have to start contributing to popular projects for free, and then later hope that one of the Bitcoin grants notices and starts funding your work.
It's part of the reason why not many developers are available for these sort of projects.
I think it is a good practice to improve your skill though. From what I can observe, good contributors do not have issues finding funding for their work. Especially those that stay out of drama and focus on building.