That has the benefit of Systemd not letting more than one instance run at the same time, which I'm guessing is one of the reasons the lock was needed in the fir

That has the benefit of Systemd not letting more than one instance run at the same time, which I'm guessing is one of the reasons the lock was needed in the first place. You just have to make sure that the type of Service used is correct, so that Systemd can track whether Restic has actually stopped running.