Supplemental update stuck on installation screen - how to break free? Mojave, Mac Mini late 2014. 

Tried to install Supplemental Update 2, but it froze. Progress bar was only 1 cm. After 2.5 hours, forced it shut, then used CMD+ALT+P+R and now progress bar slowly got to the end, filling entire bar... and stuck. 

Forced it shut again, then tried to reach both Recovery Mode and Safe Mode.
It will not go there, just keeps installing.

It has been several hours since leaving it to do it and no changes at all. Progress bar is full, but nothing happens.

At this point I just want to break free from installation screen and get to my files. Is there any way to do this or do I really have to go to technican?
In general, Apple software updates will try again and if you’re stuck, you’ll eventually need to power off the machine and risk leaving the disk in a bad state.

I would power down and try restarting and see how it goes. Worst case, use Internet Recovery when you can to install whatever version of OS you want. As long as you don’t install an older version, your data should all be intact. Command R is the usual command to initiate internet recovery when you hear the power on chime or see the Mac start to power up. 

Sometimes booting in Safe Mode (hold shift) can get past an install that’s staged, so that’s a long shot if you don’t want to seek hardware service or have a bootable OS brought to the mac so you can triage further.