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.
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)