Im actually just did that on the Live Boot with the Edit option in Grub.
Sadly, It doesn’t changed the situation.
But i could with some heavy and strange tricks, to finally get a monitor signal after 3-5 times switching between several TTY’s and Logged in and swap to desktop with alt+ctrl+f1-f6, logged in with manjaro and manjaro (in SDDM) to get no monitor signal AGAIN and repeat everything again and again and pressing also crtl+alt+f9 to reset desktop and then to TTY again and logged in.
And out of nowhere i was on the desktop and had a picture.
Its magic, thanks to @fredvej in your preview iso topic, who wrote that info.
I did switched between tty’s but didn’t used any special commands like him with run localectl set-keymap dk… which he wrote.
If someone test longer, its possible there is a clean way to get a picture. Maybe it was crlt+alt+f9? maybe the login in TTY… who knows.
But disable plymouth wasn’t the cure.