What code did you actually end up reusing?

A good question. Apart from one specialized inhouse library that I’ve reused in tens of projects, not much. I ended up rejecting writing code specifically for reuse, because its “librariness” is usually way lower than it is required for as-is reuse. And you can always just paste chunks from another project if needed. Spontaneous code reuse is a myth, imo. It’s either a separate maintained library or it’s not. Also, a set of interfaces that a library defines often adds overengineering into a project. If something is worth reusing I just extract the essence and add it to a snippets/obtf file.

Lots of code for systems provisioning, VPN creation, virtualzation, boiler-plate file handlers, pipelines, spline generation, camera and character controllers, authentication flows, infra-as-code....

There's very few times reinventing the wheel is the right decision and even if you think it is, chances are someone else on github did it better and already has a community built around solving that exact problem and you should learn their tooling instead of adding to the pile.