the most rock stable distro imo. No systemd or snap stuff. Packages are almost (if not fully) vanilla version from upstream. Simple yet efficient unix-style approach to everything like package management, slackbuilds are really good too.
Slackware gets a lot of hate, especially from the btw bros. People are spooked about having to manage their own dependencies. But I couldn't agree with you more on simplicity and stability. I've been daily driving slackware since 99 or 00, and I don't think I've ever broken something I couldn't immediately roll back and fix.
I tried to install Ubuntu on a sbc recently. And within an hour of installing this and that with all the different dependencies, I had a completely unusable system. And I had no idea how to fix it. It was totally my fault but reminded me what I love about slackware.
Slack got me through undergrad on an IBM 600e ThinkPad (which was really old even then --- around the time of the early 2.6 series kernels iirc). Great distro, fond memories.