It’s a good idea to use what you know. I don’t have much experience with btrfs but if it does what it says on the tin then it should be safe to use.

Copying the contents at the target is a good strategy. If the drives are to be put into 27/7 use later I would probably consider wiping them and run an integrity test before putting them to use, as once they start being used it will be too late (and stay as a doubt in the back of my mind).