I do pretty much the same thing. I have a custom starter kit and a custom plugin with functions I use every project. Those are mostly helper functions for small things I do over and over again.
Other than that, I rewrite pretty much everything from scratch everytime and that’s because I learn something new with each project and so I prefer to put my improved skills to use on the next site I code.
Since I use kirby to code custom websites there’s not much code reusability in general.