I invite you, respectfully, to take a moment and consider how many average people would even begin to comprehend anything that you suggested.
Android's "free" market comes with the freedom to be bombarded by ads. As much as i used to like that, i now recommend only apple phone and computers to normal people. The average person has no idea what lineageOS is, forget about knowing how to install it.
> The average person has no idea what lineageOS is
The beauty of it is that the average person can buy a LineageOS-enabled system without having to know it's not "normal Android". Look at /e/OS for instance, and hopefully soon GrapheneOS will come preinstalled on some Motorolas.
And AT&T shouldn't allow you to connect other phones, or modems at all. You should only be able to get a phone by renting it monthly from AT&T - everything else should be illegal.
Personally I did none of that on my android (except set up firefox with ublock), and I get very few ads and they're easily stopped.