Update on current progress: Poking around as to what could be an issue yields no answer, it does not seem to be hardware acceleration, or problem with X, but also Xwayland has this added bonus of somehow being able to recover from this lock and drop me to sddm, I'm including dmesg output from the first time it happened to recover.

This has been happening more and more often this May. For some reason dGPU started hardlocking system at random again, like with <4.19 kernel, freezing screen (except for mouse cursor) and often not even allowing to switch tty. Fortunately I was able to switch once and dump dmesg, which is included, if you need any more infomation please ask immediately, as it is important to me to regain stability as soon as possible.

October 2021 update: I still have no idea what causes this in the first place, 10% of lockups have been somehow eliminated by forcing Southern Islands support in boot config parameters, while 90% were gone after switching from Dolphin to PCmanFM-Qt, can confirm that, because recent accidental opening of Dolphin produced a lockup in under a minute, otherwise system is stable, which is why I've changed the name of this ticket.