I got this issue and uhm… held down the power button and the moment the screen went black I ripped out the battery, and let it cool down as it was hotter than I was comfortable with. After looking here and into safe booting, I put in the battery and then successfully failed to boot into safe mode. I think that happened 3 times. Held down power button and volume down in the black Fairphone and Android logo screen. Held down too long I suspect. I then let it boot into blue Fairphone screen to see what would happen. It booted properly and told me it failed. I initiated another try of downloading and installing (after a backup) and the longest part was the installation and optimization. The blue screen the last time was max 10 minutes and it suceeded the second time.
Thankfully it worked out easier than it could potentially be. To be able to remove battery is also a major advantage if software stalls.