If you do not want to root your device:
1. Install NetGuard or No Root Firewall to view what's going on from network perspective.
2. Install ExodusPrivacy to generate a report on apps wrt sdks in use by them.
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If you are okay to root the device:
1. Install XposedMod, and then XPrivacyLua module, and work through the options.
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If you're okay with flashing a ROM:
1. Consider LineageOS + microG
2. If you are using Pixel, consider ChromeheadOS (edit: CopperheadOS) [3].
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If you're okay with a new device:
1. Consider purchasing puri.sm Librem 5.
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[0] https://news.ycombinator.com/item?id=18788410
[1] https://guardianproject.info/apps/orbot/