It's similar with NixOS, patching a package is just adding a few lines in a persistent (and generally short) config file. You "only" pay for that patch by having to update it for newer versions and by compile time.

The developer experience isn't as good as browser extensions yet, though. Iterating on a patch means downloading that package to a local directory and building it there, which won't be enough for, say, patches to system libraries. You have to actually apply the system configuration for that, which means recompiling.

I should maybe give Gentoo a second try. I last tried it on a dual-core thinkpad and it was a pretty miserable experience due to the long compile times. These days I have fast computers, and I hear Gentoo even started shipping binaries recently.

I have a huge amount of respect for the work distro maintainers do. It's not especially fun or glamorous work, and many are unaware that it even happens, but it's essential.