So far, I've managed to learn how to quickly upload new configurations of stage-2 #mobilenixos software to a #pinephone using SSH. Not much harder than desktop nixos actually, but it works for the current boot only so far.
The procedure of updating stage-1 and kernel without re-flashing is still unclear, but I had a chance to see a recovery menu where I could select OS generations. I am sure there is a method to add new items to this list.
.. but the cross-compiler way assumes users to fix bugs in Gobject-introspecrion. Thank you again, Gnome. I think I'll try remote native builder.