I suggest that when switching to systemd to consider two running modes.
The most straightforward running mode is as a (personal) user service, which is what the AUR package is shipping with.
The main problem with this mode is that it requires that the user would be logged in or that the home directory would be unencrypted, and with systemd-homed the latter is not enough as homedir is not available (not mounted) when the user is not logged in.
The second running mode is running as a system service. Look at the
systemd.exec.5 man page to get an idea of the common conventions.
With this mode I suggest to run as a dedicated
insync user and group and to use
/var/lib/insync for the stateful dir so the config and synced folders will be saved there.
I also suggest to put the Unix socket in
/run/insync/insync-headless.sock when running as a system service, and in
$XDG_RUNTIME_DIR/insync/insync-headless.sock when running as a user service.
The insync-headless command should have a
--system options to choose to which socket to connect to, and if the user want to connect to the system service then just adding the user to the
insync group should give the proper permissions to communicate with the service.