You mentioned recreating the UEFI partition to 500MB. This is a good start, but there might be issues with its contents or how it's being accessed.
Boot from Windows 11 Installation Media (USB): This is crucial for all the following steps.
Open Command Prompt:
During the Windows setup, press Shift + F10 to open Command Prompt.
If that doesn't work, go through the initial setup screens until you see the "Repair your computer" option in the bottom left. Click it, then "Troubleshoot" > "Advanced options" > "Command Prompt."
Identify Your Drives and Partitions:
Type diskpart and press Enter.
Type list disk and press Enter. Note down the disk number for your main Windows 11 installation (where your primary C: drive is).
Type select disk X (replace X with your disk number).
Type list partition and press Enter. Identify your EFI System Partition (it's usually small, formatted as FAT32, and might be labeled "System" or "EFI"). Note its partition number. Also, identify your main Windows 11 partition (usually the largest, formatted as NTFS). Note its partition number.
Assign a Drive Letter to the ESP:
Type select partition Y (replace Y with the EFI partition number).
Type assign letter=Z (replace Z with an unused drive letter like V, U, or T).
Type exit to leave diskpart.
Rebuild the BCD for Windows 11:
Now that you can access the ESP, let's try to rebuild the BCD specific to your Windows 11 installation.
Navigate to the ESP:
Type Z: (the drive letter you assigned to the ESP) and press Enter.
Type cd \EFI\Microsoft\Boot and press Enter. (If this path doesn't exist, try cd \EFI\Boot first and look for a BCD file).
Backup the Existing BCD (Important!):
Type ren BCD BCD.bak and press Enter. This creates a backup in case something goes wrong.
Rebuild the BCD:
Type bootrec /rebuildbcd and press Enter.
It should scan for Windows installations. When it finds your Windows 11 installation, it will ask "Add installation to boot list? (Y/N/A)". Type Y and press Enter.
Fix Boot Files (Try again for fixboot):
Type bootrec /fixboot and press Enter. (The "access denied" might still occur, but with the correct ESP accessible, it might work now).
Type bootrec /fixmbr and press Enter. (Less critical for GPT, but doesn't hurt).