Wow, this resonates with me! I stopped doing full-stack development a while ago, and have since January been contributing to open source projects as a contractor (my next gig will be for ISRG, the makers of Let's Encrypt, :D). By working on open source libraries I get to tackle interesting problems without being bogged down too much by accidental complexity. It feels like the ideal solution to me (though it is difficult to replicate, as it takes quite some luck).