I have not played with Kolibri OS, but I have played a bit with Genode/ScultptOS. And I have always taken an interest in the "Microkernel and component based OS" Developer Room at FOSDEM. Start here to wet your appetite
https://archive.fosdem.org/2020/schedule/track/microkernels_and_component_based_os/
https://archive.fosdem.org/2021/schedule/track/microkernel/
https://archive.fosdem.org/2022/schedule/track/microkernel_and_component_based_os/
https://archive.fosdem.org/2023/schedule/track/microkernel_and_component_based_os/
Once you look, you will find out that the space of small operating systems is surprisingly large. I have written about small systems I'm aware of here:
gemini://ew.srht.site/en/2023/20231109-re-small-operating-systems.gmi
https://portal.mozz.us/gemini/ew.srht.site/en/2023/20231109-re-small-operating-systems.gmi