I too am a diehard Slackware user (I would say "Slack" like J.R. Dobbs intended it, but people would confuse it for the chat) and I'll echo your sentiment.

Guix and Nix are the only ones in the field which deeply identify and solve issues in package management, and are the solutions I'd be most open to using.

For personal usage, the Slackware tarball format without dependency resolution has never failed me.

(I also have an affinity for Portage, but it's a delicate thing to use.)