I've installed the latest 3 available updates, each time my phone rebooted as expected with a +60% battery and now half an hour later when it was just lying on my desk I suddenly heard the startup sound being interrupted and now it's dead: pressing all buttons or connecting the charger doesn't make any difference ?????

Very weird issue, but we can try to solve it.

Put your device under charging (use the original charger and usb cable) and wait 10 minutes.
Then, following this simple tutorial, boot it in recovery mode and restore it to factory settings:
viewtopic.php?f=192&t=334922

Let me know if your device gets back to a woking state!

PS. Can you post the version of the latest update you did?

thanks for the fast reply.

I recall it was an update that had a date of January 23th, but I can't give you the exact number since well obvious reasons
I'll give the recovery mode a try when I'm back home and keep you posted.

When I was in the recovery menu it suddenly turned off after a few seconds, that made me think it was a power problem so I reconnected my charger and then it kept running. I did an entire integrity check and display test, which both turned out ok (tip: don't use messages like installing system update or anything that's very scary/confusing when you're just testing graphics).
After those test I just rebooted and my thoughts were correct: the battery level was at 4% after it shut down 2 days ago with over 60% when I applied the GV8000Pro-Blackview-V0.1_20170803_20180124-1443 update, when I connected the charger back then it didn't give a red charging led or anything.

I'm glad it's alive again without a factory reset, but as a precaution I'm still gonna hold my galaxy s5 as a spare, hoping I won't need it anymore

Good to know, so your device is now working.
I'm tagging your issue as solved