I suppose one option would be to enable data syncing to Apple Health, and periodically delete/create new Oura accounts to purge historical health data. Not a great workflow, but would let you benefit from E2EE with Apple Health while using Oura (assuming Oura’s “delete account and all data” does what it claims)
AFAIK, they even have some watches with no radio hardware so that they can be used in sensible environments.
I wish Casio, Polar, Suunto and others provided this functionality.
There is some community software for Polar that enables offline data exchange, but it is a bit hacky, and OFC no firmware updates.
Suunto used to have a really good offline solution, but they discontinued that and moved to the cloud.
I’m probably missing something, but I can’t think of anything you lose if you don’t use the phone app.
I'm only a casual fitness tracker so the Apple Watch fits my needs better.