So, how onerous is package management in Slackware if there is no dependency checking?
I can't say that I have used Slackware ever, so I don't know. But I have used a Linux distro where every package was statically linked by default. In that case they didn't use any dependency management since every package was just a tarball of some fully statically linked executables or libraries. The only real dependency was the Linux kernel.
It is very, very rarely onerous, since 98% of the time you already have the libraries/other dependencies installed. The other 2% of the time you might be stuffing around with community-built SlackBuild scripts or writing your own, but I have never had any major dependency-related issues before myself.
(I have been daily driving Slackware64-current on my personal laptop for a year or so.)
If I found software that didn't have a package, I would compile it. When I got more advanced, I'd create slack packages for the software. Since I'd compiled it on my system, dependencies were already met. I don't remember for sure, but I believe that the tools to build autoconf software were something in the core package set.
On the other hand, I found that, often, not having to install all the dependencies, and not having the installation of a package fail because of some missing little optional dependency, is a blessing in disguise.
A couple of things that make installing software and keeping it up to date a breeze are slackpkg (built-in) and sbopkg (third-party).