I dug out my old Note 3 just to see what versions of Android are available for it. I am running a version of Lineage 13 so fairly recent. Runs a little slow though but not to shabby. Still love the size and shape of that phone.
Lineage instructions are usually the best when it comes to flashing phones. If you haven't read it already here is their OnePlus 8 guide. OnePlus seems to have a few more steps than the Pixel phones but still all fastboot or adb commands.
https://wiki.lineageos.org/devices/i...oodle/install/
They go up to Android 13 which most ROMs are now anyways and 14 is just now slowing rolling out to the latest OnePlus phones.