On my Android, (sadly without root) I use NetGuard for similar purposes. I blanket disallow google for many apps. I allow carte blanch to my personal servers for apps that I use, but any telemetry of theirs is stonewalled.
In Firefox I use containers to separate FB/Twitter to their own hole, while I blacklist them in uMatrix for every other circumstance.
That said, things like doubleclick and crashlytics are fine to be black listed throughout a network.
My thoughts were more targeted toward a properly sandboxed os that gives users the chance to control on a port/ hostname level what is being connected to.
I do both.
I run a firewall on my phone mostly to prevent applications from communicating out without my express permission. I also don't turn on my phone's radios without connecting to a VPN that I run at home, so that all of my phone's traffic gets routed through the defenses I've set up for my home network.
On top of that, I avoid using the web on mobile devices to the greatest degree possible.
I know there is a Linux (python?) client that will sync (at least some models of fitbit) to their cloud service. But I've no idea if there is one that will dump the data locally. It's entirely possible that the cloud client is merely passing along an opaque blob.
I was responding to a comment that was talking about creating Free Software to communicate with the device, specifically the idea of proxying access to the corporate server and modifying the communication, rather than implementing the whole protocol from scratch.
I'd guess the Fitbit protocol is encrypted, from a desire to keep people from cheating their activity reports. If a company wants to spend the development time, there is basically nothing that can be done to prevent a device requiring Internet access on a dumb-pipe all-or-nothing basis.
Its on my plate to make a go at it, with some inspiration from pihole. But really it'd be about enabling myself to use some of this great data without sharing it with a third party.
For example, I'd wear my fitbit if it wasn't reporting in to their servers. But if I force my phone through a VPN, which routes through my transparent proxy, I could feed fitbit junk data while scraping the pieces I want to my own system.
We need apps that take control of these devices and their telemetry away from the third parties.