I think working with a ready-made Unix isn't as friendly to the idea of 'hobbying' an OS. For studying how Unix works, it's great. But if I wanted to make experimenting with my own bespoke system worth it, I would try to not have preconceived notions about how it should work, or copy an entire design. There are dime-a-dozen Unix-workalike hobby OSes out there, and in the shoes of an experimentalist there's a lot to weigh you down in the Unix ecosystem, and a lot of things worth trying that break from the way Unix does things.

If you really did want to work with an existing OS for actual research/experimentation purposes, and you're a fan of C, I would go with Plan 9/9front. There's a reason Bell Labs ditched Unix--because it wasn't a worthy research platform anymore.