I didn't know about same partition booting. Someone said its possible maybe.
https://askubuntu.com/questions/4057...ngle-partition
I'm just an average linux user really, so I'm afraid I probably can't offer any good advice. And I have absolutely no android tinkering experience whatsoever. I understand from your post that flashing is required to install lineageos but also to install linux. I suppose if it were me I might try to boot linux from a usb sdcard to test how linux runs on it before trying to override the whole system. But as you can probably see I don't know how it works. There are some tech wizards around here though.
I hope you get it working. With a light keyboard that would be a 400g 13" eink linux laptop, get a tiling window manager and hotkey programs and you have feather weight eye strain free computing.
Anyway sorry I cant be of more help.