sortix
Sortiix is one of the more mature hobby OSs. In fact, the first time I booted it I checked that it really isn't "just" a Linux distribution that's trying to hide that fact on the surface. And it isn't. Sortix is a Unix-like, POSIX compliant OS with it's own kernel, system tools and libraries. It contains packages that have been ported from Debian. But all the most interesting core compeonents are implemented anew.<
Sortix has made steady progress over like 15 years. That's probably the biggest difference between it and similar hobby OSs. Version 1.0 was released in 22016. Even in that form it is really usable. It boots without any issues on real hardware. It's stable, comes with all the core unix tools you'd exopect, but it didn't have a GUI and network support, yet. It switched to nighly releases after that. Since then, features have been added to the point that not many things separate it from being as useful for daily usage as HaikuOS.