You are right, if Okular is your first KDE app, then it's about 256 MBs for the install. Although I still hope that Evince catches up with the capabilities of Okular...
I gave Mendeley one last chance, resetting all my web and desktop settings. It's still driving me insane, constantly trying to do stuff and being slow.
So, I don't know why I had previously kinda dismissed Xournal in my mind but I gave it another try just now, and it looks awesome. The only "flaw" is it doesn't seem to let me highlight lines in a straight way. But it can export pdf with annotations, it lets me comment, etc. So it has all that I need.
So, I think I'll migrate to Bibus (manager) + Xournal (annotator) for pdf.