Google is already pulling everything they can from relays. But yes, the event goes through Firebase servers.
Notification data will be stored on FCM servers so I would assume Google will be able to see notification data. Regarding content I guess it depends on the data within the notification?
https://firebase.google.com/docs/cloud-messaging/concept-options#encryption_for_data_
Discussion
Thank you for giving us users of both options (F-Droid & Google Play) 🤙⚡ hugely appreciated I wish this was an option for all apps