Thoughts summarised from private chat with core team members:
It doesn't make sense for LE to ceed control and its apex position. Our buildsystem is designed around our own needs, follows our own schedule, and is managed to our own standards. Our core team is small and focussed on contributing to LE, and multiple contributors flag themselves as less likely to remain engaged with the project if focus or effort levels are notably changed. Carving up the buildsystem in the way you've described is overwhelmingly seen as a compromising move.
Over the last decade we expended considerable effort to move away from vendor codebases. They are static which causes long-term package management issues as the needs of upstream Kodi and core packages like kernel and u-boot continously move forwards. They inherently accumulate ever-more technical debt over time as there is nowhere to offload and upstream changes to. Creating a pseudo-upstream target solves that issue for downstream forks and the forks of forks, but will progressively complicate the pseudo-upstream fork. Having worked for years towards eraditcating that problem from our codebase, LE does not want to be the pseudo-upstream target, or to be downstream from that pseudo-upstream target. If the community wants to support a device that has no upstream support, our belief is the community should focus on upstreaming support, which benefits the entire Linux ecosystem not just the audience of *ELEC projects.
The group of interested parties you foresee harmoniously working together on a common mission contains specific people who we collaborated with in the past, and would never want to interact with again. It also includes groups we already have an excellent relationship with, who choose to keep a level of downstream independence even when they openly recognise working with us more directly would reduce their own effort. They value being able to do what they want, however they want, whenever they want.
We are already broad collaborators: our Slack instance is ~20% regular contributors and 80% external folk we share interests with. We intentionally keep Slack invite-only to force separation from end-user support and to ensure it remains a polite and productive workspace. We will continue to invite and focus on people we established a positive rapport with.
We are not against structural buildsystem changes that improve downstream coexistence. We are not against accepting more distribution/project/device targets; adding things that are not always built is nothing new. However long-term maintanability always trumps short-term demand, so there will be limits on what we accept, and for larger changes we will need to trust the contributors. Rather than tackle large impossible topics, people need to focus on smaller code problems and technical details. We know there are issues downstream, but we don't spend our lives poking around in other peoples problems so we do not understand specifics. As the saying goes: If you need to eat an Elephant, do it one bite at a time.