The telemetry mostly comes from Google Play Services, which are optional on LineageOS. I've been using my phone without it and side-loading open source programs - no issues so far and no telemetry collected as far as I can see.
What are the biggest issues for you with removing Play Services?
Wouldn't this remove the ability to use almost all commercial / Play Store apps and leave you with what's available on F-Droid or similar?I
At that point - with 90% of apps gone - I might as well get an old BlackBerry or something and get by with just basic IMap mail, calendar, contacts, and SMS.