No, it's really not. Just go buy any of the old Pixel phones and they all work with lineage. I mean, it's not that big of a deal. You're making it sound like it's a huge effort to find a new phone, and you just have to accept the fact that you're going to have a couple of generations older to run these type of OSes.
A phone will only call out to Big Brother if it has the code to do so. It needs the code. If the phone is flashed a certain way, it won't call out to Big Brother.
It's not exactly a fun experience, but I wouldn't call it a massive problem either. More of a small headache.
As far as Graphene goes, it goes down to literal hardware chips in your phone that drive security that are only found in certain devices, typically enterprise-style devices. It's like a TPM chip in your computer.
As far as security goes, there's no competition. Graphene OS is the best, and that includes current modern OSes. Governments really fucking hate this OS and have been targeting people that have it on their phones Because there's absolutely no backdoors and there's no way to get the data off the phone, unlike a Google or an Apple device.