For the time being there's no way to force a sync of the overlay fs to the underlaying one without restarting ZRAM which isn't acceptable to the apps to run on top.
Can you provide some script to use rdist or similar that syncs mountpoint xxx to its lower fs /opt/zram/xxx.bind ?
Plus setup a systemd timer to execute at regular intervals.
For the time being there's no way to force a sync of the overlay fs to the underlaying one without restarting ZRAM which isn't acceptable to the apps to run on top.
Can you provide some script to use rdist or similar that syncs mountpoint xxx to its lower fs /opt/zram/xxx.bind ? Plus setup a systemd timer to execute at regular intervals.