NetBSD 9.3

But admittedly it doesn't seem to have quite the following of those two other BSDs.

It feels like at this point, Linux has the most choice out of the box, when it comes to software for displaying images/pdfs on the framebuffer? I use fbpdf on Linux, which is not available on NetBSD. Then again, it is based on libmupdf, which is available, so maybe I should get my (shaking) hands dirty. :)

I like it very much how NetBSD encourages user-side modifications of the kernel, e.g. for changing the console font or underclocking the cpu. Every time I read the NetBSD documentation, I feel tempted to install it, because the docs are so well structured and written. Very welcoming, even for (curious) non-CS users like me.

For that reason, using NetBSD may possibly be more educational than using Linux, in the long run? Due to heavy reliance on the official documentation, you'll have a more structured understanding of how stuff works -- as compared to trawling web forums of various Linux distros and sometimes blindly copy-pasting solutions or hacks provided by others. In that sense, a good base documentation encourages more acknowledged use and going to the details from early on. (Obviously, there are Linux distros witch excellent documentation, too, like the Arch wiki.)

That said, I am rather pleased with Tiny Core Linux. Their current release is something like 12.x, and I've been in their boat since version 6.x. It is a really simple, well thought out distro, excellent for older hardware; somewhat similar in that sense to the BSDs. But, yeah, sans that documentation. :)