secure confinement is managed by the kernel, snapd only utilizes the features it provides … if you run a dameon, that happens as root inside the confinement. per definition the kernel will only allow writes to $SNAP_DATA (/var/snap/snapname/current), $SNAP_COMMON (/var/snap/snapname/common), $SNAP_USER_DATA (~/snap/snapname/current) and $SNAP_USER_COMMON (~/snap/snapname/common) … if you add the home plug you also get access to all non-hidden content in $HOME …
if you run the app as a normal user ~ translates to /home/$USER … if you run as a daemon ~/ translates to /root only … to achieve what you want, the daemon would need to run in the users session, not as system daemon …
have a look at: