I just tried this solution again after reinstalling the bumblebee driver. For whatever reason, it worked this time so the issue seems to be solved.

I just tried this solution again. However, I changed /etc/bumblebee/xorg.conf.nvidia before running optirun intel-virtual-output and it worked this time. The Archwiki didn't mention that this must be done beforehand so I had only changed the configuration files afterwards and re-running optirun intel-virtual-output probably didn't work as there was already an instance of it running.