Fast boot needs to be off. That may not be the problem, but it is not helping. Did you see the dual boot tutorial? (Substitute Mint for windows) There needs to be a shared ESP (/boot/efi) FAT32 partition for grub to work with btrfs. This requires manual partitioning when installing Garuda. Mint's grub can't see a pure btrfs distro (and mint's grub won't boot Arch distros anyway.) Also, be aware that Mint will reset the boot order whenever its kernel is updated.
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