I can install ok and it will start up, and soon will message updates are available, and as soon as I install the updates from software manager or from console, my pinephone will give a bunch of error messages but not boot anymore.
Is KDE Plasma mobile abanoned by Manjaro, or does nobody care about?
Best is probably to do it after the -Syu and before rebooting. Or even after rebooting if you can manage to enter text mode and enter the command using a physical keyboard (since you cannot get into Plasma Mobile).
With a physical keyboard, you can probably switch to a text-mode vterm using Ctrl+Alt+F2 or the same with some other F-key.
But the easiest solution is to swap the packages between the -Syu and the reboot.
The update should not automatically reboot. Do not use Discover to update, it is just generally broken on Arch and Manjaro (except for Flatpaks). If you do the update on the CLI (with pacman or pamac) or with pamac-gtk, it will not reboot without asking you.