With the UEFI Bus Support you can find the framebuffer via the BAR (Base Address Register).

This code works for me in Qemu when the AddressSpaceGranularity is 32 bit.

EFI_ADDRESS_SPACE_DESCRIPTOR *asd = NULL;

Status = SystemTable->BootServices->AllocatePool(EfiBootServicesData, sizeof(EFI_ADDRESS_SPACE_DESCRIPTOR), (VOID**)&asd);
if (EFI_ERROR(Status)) {
    Print(L"Failed to allocate memory for the asp: %d\r\n", Status);
    return Status;
}

UINT64 count = 0;
UINT8 BAR = 0;
Status = PciIo->GetBarAttributes(PciIo, BAR, NULL, (VOID**)&asd);
if (EFI_ERROR(Status)) {
    Print(L"BAR attributes err: %d   ", Status);
} else {
    if (PciConfig.ClassCode == 3) {
        UINT8 *vram = (UINT8*)asd->AddressMin;
        UINT64 value = (UINT64)vram[0];
        UINT8 data = 0;
        for (int i = 0; i < 30000; i++) {
            data = vram[i]; // Example usage: reading byte-by-byte from VRAM

            if (data == 0) {
                vram[i] = 100;
            }

            if (data == value) {
                count++;
            } else {
                Print(u"%dx %d, ", count, value);
                count = 1;
                value = data;
            }
        }
        Print(u"%dx %d, ", count, data);

        Print(u"\r\n");
    }
}
SystemTable->BootServices->FreePool(asd);

With the structure:

typedef struct {
    uint8_t                             QWORD_ASD;
    uint16_t                            Length;
    uint8_t                             ResourceType;
    uint8_t                             GeneralFlags;
    uint8_t                             TypeSpecificFlags;
    uint64_t                            AddressSpaceGranularity;
    uint64_t                            AddressMin;
    uint64_t                            AddressMax;
    uint64_t                            AddressTransOffset;
    uint64_t                            AddressLength;
    uint8_t                             EndTag;
    uint8_t                             Checksum;
} EFI_ADDRESS_SPACE_DESCRIPTOR;

AddressMin is the starting address of the bar. And the ResourceType should be 0. https://uefi.org/specs/UEFI/2.11/14_Protocols_PCI_Bus_Support.html#qword-address-space-descriptor-2

I am currently trying to make this work too but my system is also 64 bit so this doesnt work for me except for Qemu tests.