Headphone jack:
Get a USBC adapter
Batteries:
Not easy to change, check out ifixit
Debian <> Pixel data transfer:
USB, or Wireless across any protocol you can imagine.
My experience with GOS:
Contactless payments and some QR code payment methods from banking apps in my country refuse to work with GOS.
Otherwise, I love it. Very easy to install and use. Recommended.
Do we know why they don't make ports for newer Pixels?