The issue with Ubuntu Touch is that unfortunately it's not a genuine alternative to Android, as it actually relies on Android quite a lot to function as well as it does.

It generally uses an outdated Android kernel (which is also usually not receiving security updates) and a Halium abstraction layer to access the closed source binary blob Android drivers for the phone's hardware. It also requires that it be installed on top of an existing Android install, so in all it's more of Linuxified layer on top of Android, which means it's not truly escaping the control of the Android/Google ecosystem.

PostmarketOS and Mobian are genuinely Mobile focused Linux distros that run the mainline up-to-date Linux kernel, right on the bare metal, meaning they are not subject to any influence from Google.

Both projects often collaborate and benefit from each other, they just use a different base distro (Alpine for PostmarketOS, Debian for Mobian), but that doesn't result in any wasted effort, as ultimately any new developments, drivers, or new phone support are mainlined into the kernel itself, so both projects benefit.

They're not as usable as Ubuntu Touch is right now, but they are ultimately the better solutions long-term to ensure that all the code is under community control so we aren't reliant on outdated unsecure Android components.