@laleksic
welcome to LQ , IMO Slackware is the right "battleship" for You.
Most GTK is still GTK2, build chain is as stated of the most usable out there for upstream direct builds.
Most of the system is dependable, upgrades cleanly as per shipped documentation, and the clean system is rock solid.
Using "own software" form ~/* or from /usr/local/* as the older
Slackers are accustomed to do, will surely never break the system.
Besides the build system Slackware is renowned by, it has also a flawless track record of binary compatibility among the distros that "matter"
Both XFCE and KDE shipped by Slackware are reliable, rarely glitching and thoroughly tested before picked for inclusion. While i rarely found the shipped components of KDE attrcative for actual work (and God knows i tried) there are some actually useful productivity apps. I personally rely on LibreOffice from AlienBOB's repos and "just work"
Things are looking much more bright for Slackware than ever, there are literally tonnes of preuild binary packages available across the web, even more so as "recipes" called SlackBuilds.
I found my "gray havens" with sbotools-a ports like system tailored for Slackware.
All in all give it a shot, and see for Your self -only thing You could regret is - You didn't find it earlier