These, “just change the firmware” comments are silly. I should get basic phone functionality from my phone out of the box with exactly ZERO effort.

I can use an iPhone without an Apple account. I cannot use an Android phone without a Google account unless they changed that in the last few years.

Both Android and iphones require an account to download apps I believe. Am I mistaken here? Other than that they can be used in the same way without accounts. In fact, with Android an email account suffices and giving your phone number is "opt-in". Apple forces you to give your phone number to create an account last time I checked. Also no F-droid on iphones? But the iphone is the better "phone" experience because there is less bloat and UI nagging.

The other aspect with setting up third party firmware as a general "don't like the stock OS? then do this" option for many is besides the big headline limitations like safetynet/attestation, it also either involves a benevolent third party setting up and maintaining builds for your device (hope you bought a popular model) and any changes match what you want, or individuals doing so themselves

You can, but you don't get access to the app store.