To be honest I don't understand how this works e.g. even with LineageOS' PrivacyGuard I still see some background processes in cache that had no permission to run on boot.

But it's absolutely necessary. I can't upgrade until I have that. I want full control of what runs on startup and what should be allowed to keep running on background (manually closed by me, I'm not asking for a skynet AI to guess).

Another one is blocking internet access but that will never happen because of their business model and ads. I guess I'll have to risk my personal data by using an open source firewall (not like I'm not going to review all the code) and unlocking bootloader/root.

Android is a disgrace because of these "small" things that they refuse to fix. But I'm glad it's slowly getting better.

This is largely due to a limitation of Android itself. If your app wants to do any sort of background task that's not tied to Google Cloud Messaging, it needs to setup those background tasks. When your phone reboots, those tasks are cleared. Many apps use the "run at startup" permission to receive a notification from the system when startup has completed. It doesn't actually mean the app is running in the background.

Oh, I see. I was under the impression that these "Run at startup" apps were using the opportunity to check-in, report telemetry, and the evil ones to start GPS tracking.

At least for the last you can go into Settings->Location and either revoke location permissions entirely or at least make sure they're set to "Allowed only while in use".

After startup\reboot, I've resorted to going through all the apps and clicking "Force Stop".

You have - it's in the apps' settings screen called "Allow background activity".

Searching for "allow background activity", I see Android 8\Oreo has https://www.neowin.net/forum/topic/1367118-oreo-allow-backgr...

I am on Android 9\Pie, and from an app's settings, clicking 'battery', then clicking 'background restriction', I can choose from 'app can use battery in background' or 'restricted'.

But what I originally asking for control over is running at startup.

For example: go to an app like Lyft's settings, Permissions, then "..." in the top-right, and select "All permissions", finally towards the end of the list of all permissions is "run at startup". What I want is to enable\disable "run at startup".