If you want a simpler system that is easy to understand, NetBSD is a pretty good choice. The source is almost entirely in a single repository for you to read, and you need a single command to build a boot-able release (you can even build it on other OSes and architectures). It's a level of familiarity with the inner workings of a system that is hard to express.

It has pretty decent support for ARM boards so that's the best hardware for trying it out, or not-too-recent x86 hardware. Another use case people have is using esoteric old machines like m68k and VAX, which NetBSD still supports with release builds.

Using it as a daily driver (I do) requires some tinkering on the initial setup, and people have mentioned it gives a vibe of using Gentoo, and that certainly has a market. But it's very easy to keep using the same setup you've had for years.

Most proprietary software doesn't support NetBSD, but if you were the type using primarily FOSS anyway, you might have all you need.