Thank you for the feedback!
Have you tried the following command? Run as Administrator in a powershell terminal,
set-vmvideo -vmname <vm-name> -horizontalresolution:<2560> -verticalresolution:<1600> -resolutiontype single
set-vm <vm-name> -EnhancedSessionTransportType HVSocket
Replace <vm-name> <2560> and <1600> with yours.
Also, the modification of the GRUB is needed.