Custom resolution on a dummy HDMI

Hello,

I’m trying to set a custom resolution on a dummy HDMI (it will be used as a fake HDR screen in 16:10 1280x800 to stream on the Steam Deck) but all my attempts have failed.

The computer with the dummy HDMI is a tower with another screen (2560x1400), I use this dummy so I can use HDR (my main screen is not HDR).

On Windows I can select this resolution but not on Linux.

I tried xrandr under x11, kscreen-doctor, wlr-randr under wayland, i even tried to add a line to grub.

Recently i tried to modify the EDID on the dummy but i have this :

This is read-edid version 3.0.2. Prepare for some fun.
Attempting to use i2c interface
No EDID on bus 1
No EDID on bus 2
No EDID on bus 3
No EDID on bus 4
1 potential busses found: 0
256-byte EDID successfully retrieved from i2c bus 0
������ �0�sAx
�t�WL�# HL���@��:�▒q8-@X,EĎ�0�pZ��X�Ď!�HDMI4KHDR
   �;F�<
Looks like i2c was successful. Have a good day.
      %h�[�� "]_`abdef^c;       P?�W_~_~g{Y�On
�▒q X,% �+[hamtarotaro@Nostromo ~]$ ^C        �< �g�]�x��������

My goal is to stream to the steam deck from my big computer (I7 6700k - 16gig Ram - Nvidia 3070ti) by any means necessary, this computer is only used for games (i have a laptop with ZorinOS for the work).

Can someone put me on the right direction please ?