phonetone please don't take this the wrong way, but your posts come across a bit rude and privileged to me. Us users are in no position to demand niche features or complain about the lack thereof. GOS is an incredible project that gives us control and agency over our data, plus it's free, open source and donation funded. I'd be happy for the time devs and users invest and try to help me understanding more details so I can make better informed decisions.
phonetone This is the sort of problem that could be easily dealt with if I had a Linux-based phone not based on AOSP.
You could try that route, but with your threat model (at least how I perceive it) a Linux based phone will under no ciecumstances meet your security standards, thus it's not an alternative and doesn't make sense to mention. You said yourself you're only allowing GOS on a smartphone.
phonetone The only way to opt out to data collection at startup is to deny net access to those Apps. Which means I have to reenable net access each time. It's very inconvenient.
Which is a setting you wouldn't have on most other AOSP based systems. That's part of why it's inconvenient, AOSP expects apps to have network permission enabled by default (when/if they ask for it).
I understand the frustration, you wish to be private and companies are getting out of their way to not let you exercise this basic right of yours. The root of this lies elsewhere though, and often that means privacy and convenience are opposing each other and you have to make an informed decision.
What you can do for now is creating an issue or pull request on Github, but it would probably be of low priority for the GOS devs because it's something niche that can be solved more inconveniently already. You could also ask AOSP developers to add the desired setting to the background permission, I'm sure GOS will adopt it quickly after it's available upstream.
Until then you'll have to rethink your threat model or bite the bullet and go the inconvenient route you already discovered. Or you could change something more fundamental, like finding a way to not use the mail app or live without notifications from said app.
I'm not speaking for the project btw.