For Slackware itself, you install all available software up front. That way, all dependencies are resolved.
You then just hide the stuff you don’t need from your DE using its menu editor, or ignore it.
During an update, the package manager updates all installed packages, installs all packages that were added to the repo and removes all packages that are obsolete.
For additional software, there is a semi-official repo that’s very similar to Arch’s AUR.
And like the AUR, it offers several helper scripts and additional package managers that do dependency resolution.
Or you use Flatpaks.