I thought I tested thoroughly but …
pfffhh!
I would have sworn I tested this but maybe I was so happy that my test device booted up I forgot.
And (almost) nobody noticed.
In v0.4.1 I fixed the boot loop but the path unit (responsible to register/start the triggered update) was not started after boot.
I recognized only now after a frozen device reboot.
So back to systemd and finding that my chosen start (WantedBy / After) criteria were plain wrong. One was just mistyped and the other can only be used within user unit.
But now I am pretty sure it is okay 
So v0.4.2 out in the wild and ready on openrepos.
P.S.: thanks for the thanks,
this was something I really really was keen to solve.