Convert Ventoy Stick to ISO?

[Schlumpfpirat]

I have already tried modifying the GRUB bootloader in partition #2, but I think bootable ISOs in general are only allowed to have one partition (might be wrong here)

This would mean that the EFI & BOOT folders would have to be moved to the primary partition and reconfigured in order to boot using an ISO. But couldn't find the appropriate RFC explaining the requirements and what's possible in general.