[SBC] [Debian 11] Feeding video through external display with Overlays

I'm trying to connect a compatible display to my RockPi 4B+ which runs Debian 11. I connected the display, got the touchscreen working, but i cannot figure how to feed video through it. I've been told to edit 20-modesetting.conf, but if i do, system won't boot anymore, forcing me to revert changes directly on the uSD from a VM. I've been asking everywhere for help without success.

Some user told me to follow this tutorial: Tutorial

And it somehow worked but not fully, as display has two colors (white and black). When i hold my finger on the touchpad, a red color will be filling the entire display line by line.

Pictures will be uploaded for better explanation

1-2: Board and display 3: 20-modesetting.conf found at /etc/x11/xorg-conf.d/ 4: Display after enabling waveshare overlay 5-6: After following the entire tutorial from the link, this is the error that appears on boot. Keyboard and mouse are locked, so i can't do anything there.