Okay, you've lost me. What is your core objective?
Grapheneos aims to be the most secure phone out of the box. That means the least amount of risk surface out of the box. That means all the control to the user.
To accomplish this mission, graphene OS uses Pixel phones. Because they give the most control.
If you want to encourage other developers to make other phones, that's great. I actually support that. I'm looking forward to postmark os becoming mature.
For you to determine what vendors to trust, you have to have a good understanding of your personal risk model. What your threats are, and what you're willing to trade to mitigate those threats. By default, out of the box, there is no trust for any vendor in gos.
You as the user have a blank slate, a locked down phone, with minimal risk surface, and no preconceived notions. If you want to install the Google store, you can. If you want to use f Droid, you can. If you want to install apps directly from GitHub from developers that you trust you can. You have total control. That is what GOS gives you, total control