Your welcome, I have not used it myself. Check out the video by Techlore. He has a older one talking about it. Mozilla has also an article about it where they testes smartwatches. I would by it via cash anonymously. If the need an email, use an alias email service. If you might want to try the app, you can check the permissions on GrapheneOS stop all you do not need. If it needs Google Services try a different profile on grapheneOS and have a VPN running. You can also try to use the watch without any apps or programs. Depends on what you need. Noami Brockwell from NBTV said she will soon release a video series about fitness tools. Maybe that will help. Let me know if you have something to share, also planning to get a Garmin next year