

It’s very unlikely that most states will use a truly open source solution, especially when big tech makes it so easy not to. Case in point: https://github.com/eu-digital-identity-wallet/av-doc-technical-specification/discussions/19#discussioncomment-13964507
I’ve been using GrapheneOS for at least 5 years now (still remember the days when their sandboxed Play Store would need a force stop after every app install 💀) and I’m a much heavier user than you (social media, YouTube/podcasts, etc.). At this point I would not use anything else.
You won’t have issues with app compatibility except for certain banking apps and Google Pay, if you choose to install the sandboxed Google Services. If not, you’ll have issues with some apps not sending you push notifications (for example, Discord and Proton Mail) and other apps not working entirely (like Snapchat). Still, the last time I tried going fully de-Googled I remember most apps still working well enough even without Google services. From your post it sounds like you could easily live without it.
Sadly the newer phones have no headphone jack, and you definitely should not go for older ones that don’t recieve security updates. And the Pixels have never had removable batteries (you can check out iFixit for more info on repairability). Any Bluetooth headphones will work. They do have a 3.5mm jack to USB-C adapter but I it never worked for me.
Data transfer between Linux and Android has always been dead simple regardless of the OSes involved. Just plug the phone in and tap a couple of things, the phone’s filesystem will show up on the PC like any USB drive.