"On all of the other handsets the Google Play Services and Google Play store system apps send a considerable volume of data to Google, the content of which is unclear, not publicly documented and Google confirm there is no opt out from this data collection. LineageOS collects no data beyond this data collected by Google and so is perhaps the next most private choice after /e/OS."
So the problem is Play Services and the Play store. Note that /e/OS uses MicroG[0] to replace Play Services so you can still use the many Android apps that require it. It's a really cool project and I think it's amazing how someone is like "Damn, I need a Google account for this app? I'll just write my own account manager to replace Google's!". Really that's the spirit of open source, and I hope more people become empowered to be able to solve problems that way in the future.
I think you can also use MicroG rather than OpenGapps on LineageOS, though I haven't tried and haven't read anything about it. Does anyone have some info about this setup?
[0] https://edevelopers-blog.medium.com/microg-what-you-need-to-...