spiral There's no guarantee that you can prevent Google from exfiltrating anything since IPC is a thing (I assume you're using Play services w/ network), but in all likelihood they won't do that. They don't need to anyway since a considerable amount of users will be using their service to back up recordings.
The point of getting Google Recorder to work was to show that GrapheneOS can leverage the same hardware features as stock OS, and it can. Sadly I'm not aware of any alternative at this point (that works offline). Whether you trust Google enough or not to use it is up to you. :)