Thus far, https://postmarketos.org/ has seemed the most likely "real Linux" distro for smartphones and tablets.

If this Mobile NixOS can get solid support on as many or more devices (including using mainline Linux, with no closed drivers, but possibly closed blobs for now), that will be another option.

BTW, one thing I don't want to see happen to any of these distros is for PinePhone to be the only actually solid device. Tempting as that device may be. If the distro doesn't run well on many devices, that's not a good sign for long-term viability, and it's also potentially awkward.