Thanks to all here and I wish you all a Happy New Year!
The solution finally was so simple: I changed from HDMI to Display Port and everything is fine! No more problems. Maybe something was wrong with the cable. And I switched back from Manjoro to Endeavouros.
Upgrading these two is difficult to a newer release. Sometimes you just end up have to do a clean install.
I prefer rolling release approach personally.
MX is my preferred choice over Mint. I’ve been following their development and tried the distro in virtual machine. Quite liked it.