There is Dahlia OS which looks fairly promising but development takes time and it is only a small amount of people.
I like the idea of the OS being more embedded focused like Android and Chrome OS but I don’t want Chrome or Google.
Is there anything else I should look at?
I’ll look into it. I just like the idea of a very small base based on Buildroot or similar.
I think Vanilla OS might then be what you’re looking for. They’ve built up their immutable OS on several layers, and while most people will want the full desktop OS, if you want a very small base, maybe their Pico image is what you’re looking for? You should even be able to build on top of it to create your own, if you need to.