PostmarketOS seems the most promising effort for a truly open and affordable handheld OS with mainline kernel and other good practices.
And there's also a useful incremental path for evolution, that starts with a familiar GNU/Linux and moves gradually towards handheld tweaks (UI, power, devices, apps).
It's important to be upfront that PostmarketOS is not yet viable as a daily driver, or people will feel they wasted their time looking at it. What it really needs is programmers, like the earlier Linux ones, who will power through the pain of getting things working well, and stick with it for months, as a labor of love.
PostmarketOS reminds me a lot about the approach Openmoko (https://en.wikipedia.org/wiki/Openmoko) took way back over a decade ago, except that they did have to build the hardware as well.
There've been several such prominent open source handheld projects in the last 20 years. PostmarketOS seems the most viable, IMHO, because it emphasizes the old Linux (and GNU) principles of getting working on lots of available hardware, with true open source, mainlined. (Separately, it also potentially builds on a wealth of GNU/Linux stuff that there's no good reason shouldn't have a lot of commonality between desktop and handheld.)
And there's also a useful incremental path for evolution, that starts with a familiar GNU/Linux and moves gradually towards handheld tweaks (UI, power, devices, apps).
It's important to be upfront that PostmarketOS is not yet viable as a daily driver, or people will feel they wasted their time looking at it. What it really needs is programmers, like the earlier Linux ones, who will power through the pain of getting things working well, and stick with it for months, as a labor of love.