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).
Yeah, your data is taken hostage. When cancelling the subscription it’s a good idea to delete everything manually before the account expires. Even if you can’t guarantee Dropbox haven’t just flagged the files as deleted, it signals intention. I did this when migrating to pCloud and haven’t received any reminders from Dropbox.