since your goal is to improve your backend skills, i think the best advice is to find a Free Software/Open Source project that needs work on their backend. for

since your goal is to improve your backend skills, i think the best advice is to find a Free Software/Open Source project that needs work on their backend. for my own projects i discovered the same thing you did, that most projects just don't need any complex backend work. i pretty much stopped backend work for my projects a decade ago, because all the interesting parts of the projects were in the frontends. contributing to FOSS projects gets you out of that dilemma, because you get to work on projects that are larger than what your personal one-person projects usually end up being, and thus have much more complexity that require a custom backend. to earn a bit of money, look for projects that offer bounties to solve specific problems.