I think people overestimate the future smartphone market by large.
The by far biggest segment in Android driven technology is the entry and mid price level. Tablets are not worth to mention in comparison to the iPad.
If Graphene wants their OS to spread largely and offer the OEM a good revenue of his efforts, they need to place a mid price to low price model that serves the basic needs in an acceptable way. That means, acceptable screen, camera, speaker and a chip set that allows browsing and apps to run meeting the average expectations of 2025.
Foldables or Flagships would mean that Graphene could stop further development, as people who buy Flagships want all the gimmicks modern spy OS offer. There aren't many under these costumers who would want to install an OS that does provide zero of those integrated features.
We have a worldwide recession and AI will kill lots of jobs in the near future. Money is tight and this is probably the only and maybe last chance to break the dominance of Google OS and iOS.
It needs an affordable phone for the masses, something reliable, something that catches all those disappointed with the intrusive AI and surveillance, that will become important soon.
A 800-1500 dollar phone and no further Pixel support (if that should happen? It's likely to happen, mostly because Google will suddenly stop producing Pixel phones, like so many other products they had got canceled) would be the wrong price model at the wrong time.
The average Joe will not spend 500-1000 dollar for privacy, when they can get a decent android phone for 200-300 dollars that "does the job".
Money matters and if you want to expand these days, you need to be a mass product. If Graphene wants to become bigger or survive without Pixel, they need to aim for the masses.
An OEM won't build a phone meeting the needs for a small number of enthusiasts only.