gnu/linux is ironically a fair point in this context. But to uhm ackshually you, chromeos is technically based on chromiumos which is technically based on the linux kernel.
Fair but Ubuntu Touch is not mainline Linux. It's a hack to get a GNU/systemd Userland working with an Android kernel (which arguably is also a huge feat)
Sure, but PMOS is by far the most complete and "daily-ready" mobile mainline Linux distro out there. In fact I'm pretty sure both Mobian and MoA use the PMOS kernels...
I haven't used postmarketOS, but I don't see why it would be any better than Mobian or Manjaro (Manjaro might not be the most stable though). Maybe you are talking about Android phones, in which case you are probably right - other distros might not support those so well. postmarketOS and other distros don't use mainline Linux, so I don't know why you would call them that, though. For me this is the biggest flaw of GNU/Linux phones.
I know that postmarketOS developers contribute a lot in different areas, but so do Mobian developers. I think the kernel we use was initially developed by Megi.