Do I retire or remove Garmin watch from my account for someone else to try out?

Retired device cannot be revived, so you'd need to add it again anyway, so removing it, performing the Master Reset on it before you borrow it, is probably the best way. Do not forget to make another Master Reset when you get the watch back. You can then add it again to your GC, and reconfigure it. You should also make a backup of the device storage before resetting - it will help you to restore the original settings, after the reset:

See the details here for the backup:

• Can I Back up My Information and Settings on My Garmin Device?
• Backup Personal Settings Instructions for Outdoor Watches