For Intel, there's DVMT that can be set to different values; with setup_var I can change the memory allocation for Intel UHD 630 from 256MB to 1GB.

I wonder if a BIOS modification could be done to go past the 1GB limit? It sounds like the size modification has to come from the BIOS or VBIOS firmware.

For Intel, some BIOS setting names that control memory usage are "DVMT Pre-Allocated", "DVMT Total Gfx Mem", "Above 4GB MMIO BIOS assignment", and "Aperture Size". If I had a platform I could re-flash the BIOS easily and had any idea what I was doing with a hex editor, I'd see if "DVMT Pre-Allocated" or "DVMT Total Gfx Mem" had an obvious memory limit number, raise it, and check lspci in Linux to see if anything happened.