Hi @pueffl welcome to the community.
Unfortunately there was another mate with same issue he did a fresh install and this work.
If you see in your output your external monitor is not listed not showed at all. Looking in the internet i did found this post what looks like to be related with your issue this is actual and they found a workaround while nvidia fix the issue. Try read the whole post is very informative and i think this fight good to the problem than you did describe.
Other post than can show some help too.
https://www.reddit.com/r/Fedora/comments/do01lf/fedora_30_external_monitor_not_detected/
I hope this can help you a bit.
Regards.