If you're serious about this: make it preemptive. Handle interrupts in the kernel and find a nice way to pass them on to user processes. Any kind of prioritization mechanism will require preemption. I've written an experimental OS myself and I know this is hard stuff but if you can't crack that this is likely to be DOA. If you just want to play around then that's fine of course, there is nothing to stop you from doing that but if you want to see any kind of external adoption it is more or less a must.

Also: this is more like an app itself than an OS, for it to be a proper operating system you'd expect at a minimum memory barriers between applications. In that sense the bar for what an OS is has been raised quite a bit since the times of CP/M where it was more of a 'handy library to do some I/O for me' (on mainframes and mini computers there were already proper operating systems back then but on micro computers we had to wait until OS/9 to have something similar).