Huh? Apple and MS have a record of communicating in sharing data with US government - plus as companies providing closed source software they are likely to introduce backdoors ... also something you might have heard of since Snowden.
If Apple actually did have a history of, say, pushing customized "iOS update" binary blobs at USG targets that undermined all the security features they describe in their white papers and in their (other) marketing, that would actually be entirely relevant to claims about the security of iOS, would it not? Even without such on-device updates, a system like iMessage can potentially be thwarted if Apple itself decides to cooperate with MiTMing, at least in certain scenarios.
(Btw I'm not actually endorsing OPs point that Apple is too close with the U.S. government. That does not seem accurate given the public heat brought down on Apple by entities like the FBI recently. But if it were Microsoft, there would be a point there.)
That's all I'm saying.
So, you flash the phone with your own build? Most phones on the market rely on proprietary drivers only available as binary blobs. Even if you find a phone that has all Free drivers, can you replace the bootloader with your own trusted binary? Even the bootloader isn't nearly as dangerous as the baseband processor, and I don't know of a single phone with a Free baseband OS.
Sorry, as much of a fan as FOSS as I am, with your threat model, every phone on the market is equally bad. This might change once we're able to eliminate baseband processors from smart phones, but even then I wouldn't hold my breath.