The librem looks nice but if I'm not misunderstanding, it's still pretty much vaporware at this point. I sincerely hope it reaches its goals though.
Next version of AOSP will have a smaller userspace.
No, you can't. Not when more and more phones ship with locked bootloaders and a smaller and smaller percentage of phones actually work well with some AOSP spin and none of the apps you use work without an unrooted, closed-source-Google-API-infested phone.