TempleOS is a very interesting OS with many original innovations, as mentioned in the "constructive review of TempleOS" (which you can look up).
However it is a ring 0 only OS, with all the consequences that implies. You can make irreversible mistakes in this way, especially outside a VM.
Other OSes worth looking into are:
- Redox OS (Rust)
- MenuetOS / KolibriOS (x86-64 Assembly)
- Haiku OS
- GNU Hurd