NitroPad T430: No Xorg since Debian 13 with Kernel 6.12, intel_iommu, grub-coreboot, heads

I am a happy user of a NitroPad T430 with Heads-v1.4-2 running Debian since years.
The upgrade to Debian 13 brought linux kernel 6.12 which works without using Xorg. Starting Xorg is not possiple and shows in ~/.local/share/xorg/Xorg.0.log the error:

Failed to set CTM property: -22

Starting Debian 13 with the old 6.1-Kernel from Debian 12 works fine using Xorg.

I could imagine that the Xorg issue belongs this line from dmesg [1]:
[ 33.604] Kernel command line: root=/dev/mapper/crypt-root ro intel_iommu=on intel_iommu=igfx_off intel_iommu=on intel_iommu=igfx_off

Editing /etc/default/grub and update-grub2 seems to be ignored, so changing there the intel_iommu-Parameter makes no sense.

In apt I find the package grub-coreboot. I do not find information if this package is helpful in my case.

According to the Heads documentation on the nitrokey websites [2] does it not makes sense to update the firmware beyond v1.3.1 for Debian systems on NitroPad T430.
But in the release notes of Heads v2.3 is written “ifgx / i915 can now be properly initialized by the OS. this … allows using the igfx properly.”

What needs to be done to run Debian 13 using the standard kernel on the NitroPad T430 with Xorg?
How to change the intel_iommu-Parameter I see in dmesg?
Do I need grub-coreboot?
Could an update from Heads-v1.4-2 to the current Heads-Version help here?

[1] Gentoo Forums :: View topic - startx Failed to set CTM property: -22
[2] Firmware Update v1.4+ - Nitrokey Documentation

Hello, you should first update your Heads BIOS.

If your current version is maximized then you should be able to internally update it.

Do you have the update feature in your Heads menu?