How can I switch from dual booting to only using Linux Mint without losing my Windows data?

A good approach is to copy your important files to your Linux Mint partition or save them on an external drive. You can then delete the Windows partition and use GParted from a live USB to resize your Mint partition.

You'll need to reinstall your games, but for your files, transferring them to an external hard drive or SSD is a solid option.

You could set your GRUB bootloader to default to Mint, which will still allow you to boot into Windows whenever you need to. Just leave everything else as it is for now.

You can keep things as they are for now. If you want to shrink the Windows installation instead of deleting it, that's another option.

To make the transition smoother, before switching, boot into Windows and move all your important files to a designated folder, like *C:/MoveToMint*. Then boot into Mint, create a corresponding folder, mount the Windows drive, and copy everything over. After verifying the files are copied, you can consider what to do with the old Windows partition.