I think firmware updates and even map routes can be uploaded offline by mounting the watch as a USB mass storage device?

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.

Go through the Garmin Express desktop app for firmware updates.

I’m probably missing something, but I can’t think of anything you lose if you don’t use the phone app.