Dual boot with Windows

In this case since they are using two separate disks, each disk will have to have its own EFI partition anyway. Normally I would say installing Windows first is a must, as you have said, but in this case it doesn’t matter.

Your Garuda might be a little messed up from getting installed in legacy mode. I guess I’m not completely sure how to undo the damage. You can try chrooting into it from the live session with your Ventoy stick and try to reinstall grub, like this: Grub fails after update - #3 by librewish

EDIT: Make sure you turn off legacy boot before you try that.