As sibling comment says, PinePhone works like this FOR NOW. OSS version is being built, but like a few things on PP "not quite ready".

When they are ready, get one. They'll be amazing no matter which OS you end up with.

I think that's how the Pine phone works. All of the components and software are open source, except the cellular radio.

With regular phones, the radio stuff is pretty darn separate, too (and in case of Snapdragon 865, on a separate chip).