I second this point, a fresh install is definitely the way to go.

Step 1: make a backup / clone the disk

Step 2: double check the backup

Step 3: Assuming you’ve got a grub bootloader, boot into Linux Mint, use GParted or Gnome-Disk-Utility

Step 4: Identify your NTFS Windows 11 partition, the utility should show if it is mounted or not (it should not be mounted unless you added it to /etc/fstab

Step 5: resize your Linux mint partition (ext4fs), & make sure you don’t accidentally move the partition

Step 6: sudo update-grub to remove the entries for Windows 11 since it doesn’t exist anymore

More info on if you’ve got an HDD vs SSD, MBR vs GPT partitioning, or a screenshot of your partition table from either of the disk utilities in step 3 would help us help you

I would recommend cloning the entire disk to another disk of equal or greater size before, best procedure is to boot to a USB installation, run Gnome-Disk-Utility, create a disk image onto a second larger disk, then restore that image to a third disk which is equal or greater in terms of capacity to your bookworm disk, then unplug your orignal bookwork disk amd then attempt to boot from that third disk (fingers crossed)

If you’re comfortable with the dd command that’s another route to take but if you’re not paying attention you can very easily wipe your own disk!

I have the exact same workflow except I have two images: one for legacy/MBR and another for EFI/GPT – once I read your post I was glad to see I’m not alone haha!