Likewise, what firmware is running on that phone's baseband chip?
Your plans aren't foolproof, but you're on the right path.
What is at issue is the wholesale collection of our data by Big Tech without our full permission and proper disclosure, and that includes the end uses to which it is put and who has possession of it. The other key issue is the atrocious way governments have abrogated their responsibilities in these matters.
I agree with you that firmware on the phone's baseband chips cannot be trusted (nor any other binary blob)—nor can Intel's secretive and all-pervasive AMT on its PC/CPU chips for that matter. (In my opinion, surveillance via embedded firmware together with what actually goes on in firmware and cloaked from view by the compilation process is serious matter that all too few bother to consider important—remember the VW emissions affair and you'll get my gist.)
That said, I accept that some 'spurious' data does escape through my systems—even LineageOS has Google addresses in its kernel—but that's not the point which is essentially that I am effectively obfuscating and disrupting the collection of my data to the point where what's left is essentially worthless to others. A check of the data and IP addresses that pass through my routers shows that my efforts have made an enormous difference (in effect, I stop well in excess of 99% of unwanted traffic by using a combination of quite different technologies). Combine this with the fact that I use multiple different browsers which also report randomized browser user agent IDs to reduce browser fingerprinting, and it turns out what I've done is quite effective. For instance—except for when I'm testing, etc.—I never see ads on my mobiles or desktop PCs and those that I've accidentally seen have never been target at me as a direct consequence of my circumstances or interests.
However, the bigger issue for all of us is that all-encompassing surveillance, whether it's by the State and or Big Tech, reduces both our freedoms and autonomy to act as humans. That's why we must resist it.
In addition, my smartphones use LineageOS without any Google Apps (GApps) installed and their browsers always have JavaScript disabled. Right, I have absolutely no need for social media, Facebook etc, and I never use any of Google's services such as Gmail—except perhaps search via an anonymous metasearch engines such as DuckDuckGo.
So how do I actually communicate? I use a separate dumb phone that has no internet access.
Oh, BTW, I never order stuff online via a smartphone, nor do I use smartphones for email.
So, am I a Luddite? Definitely not, I've always been a high tech worker, I was an early adopter of mobile phone technology, and I had one of Motorola's "bricks" the moment they came on the market decades ago. Moreover, if you saw the complex configurations of my smartphones and the ways they have been rooted (with the latest Magisk super-user software, LineageOS and firewall configurations to stop app software phoning home) then you'd realize I'm more a mobile communications platform than a person with a smartphone).
You may well ask why do I go to such lengths. It's principle really: as I see it, when I bought my first Motorola "brick" the telephone service was essentially anonymous with the option of users being listed in the telephone directory. Since then governments have deregulated and sold out our phone systems to Big Tech without any of us users ever having had a say in the matter—the damn hide of them! Nowadays, not only has Big Tech (Google, Facebook, Microsoft etc.) usurped our internet but they've also fucked our phone networks by actually integrating their corporations' businesses into our telephone systems—and they've essentially done so without anyone's permission.