Practically speaking, I would think that Google's data is even more susceptible to be found in court and used in that manner than data held and passed on by smaller, relatively unknown apps that lawyers may not even know are installed on locked phones.
You have a good point about there being a larger surface area and more weak points when more than one entity handles data, but I'd also be wary of heavy-handed regulations along that dividing line. I don't think we need to give Google monopoly rights to another big chunk of the ad industry. If a user might prefer an entity other than Google handle certain pieces of their data (maybe they strike a more transparent and rewarding deal than Google), they should have that choice.
[1] https://theintercept.com/2015/11/06/how-law-enforcement-can-...
As long as your Google account remains secured and no one manages to sneak a 2nd Google Account on your phone, that is. Find My Phone is a real time tracker that can be abused e.g. by spouses or stalkers. IIRC only triggering the "audible ring" leaves a trace on the phone (a notification + the obvious sound), to check if someone used FMP for tracking one has to dive deep into the Google Activity Logs.
Only way to protect yourself is, of course, to disable GPS on the phone - but what if you want to use Maps or another navigation at the same time?
The recommendation that you could disable GPS is pointless because if they could sneak a 2nd account on it they can also re-enable GPS.
Cell phone companies have your location regardless of whether or not GPS is enabled. You don't even need a smart phone.
Not true, a notification is shown whenever the device is located.
Though if someone can "sneak a 2nd Google Account" onto your phone they can probably install something more nefarious and less observable.
So, the fact that the providers are keeping tabs on you and storing all that metadata for some government agency (cause you might be dangerous!), just means that they have discovered a way to recoup some of the investment costs. Thank your government, because the phone companies likely wouldn't have been this creative by themselves.