It looks I have shot in the right way. Just finished du right now.
After half an hour of awful diving into kde5 dependency hell, manually installing countless qt/kf5/etc packages (like I were on slackware), I finally unlocked the "592 hell". Nothing to do with deb-multmedia (I run it since years without glitches), all offending pckages resulted from main debian repo. Btw, although k3b is installed (no kdenlive, though), it wasn't one of those packages calling for those unsatisfied dependencies (all are listed as ">= some version" so nothing holding back to 592; same scenario of k3b from debian repos). It runs in fact cleanly with 5.10 stuff.
Anyway, still unknown why a simple "apt update" could sort out such hell, and, especially, why following updates never solved it. This is the real bad part of the story and I have no clues about it. Just hoping it does not happen again!
And, btw, please leave God outside this topic, He doesn't deserve your swearing.