Not OP, but I'll pitch in because I'm an old Slackware user (late 90s to late 2000s).
Slack was as close as you got to the system you would get if you compiled everything from scratch. Basically, you just had Pat do the package selection and compiling for you. Even the package manager was really just a script to compile the source package. That made for a really clean, simple setup and made it easier to get support for software directly from project forums or mailing lists. If you had an idea of the dependency graph, it was dead easy to troubleshoot because there was no package maintainer making choices that you (or the developer) didn't know about, which cannot be said for Debian.
I really liked it, but Linux got more complicated and I got more busy and eventually I just wanted a system with sane defaults and a choice of DE. Slackware is very opinionated about its package selection and developers have shifted to the reality of systemd so now it's sometimes more of a hassle to get a package working out of the box. As apt improved, I also had fewer issues with Debian, though its sometimes out-of-date repos are frustrating because apt makes compiling from source more complicated than it was on Slack.
If you want the Slack philosophy of close to upstream but the convenience of a modern distro, I think Fedora is the good choice, but I have a soft spot for Slack.