Your first bucket idea would work with some slight modifications. Instead of using two pumps, just use one. Put a return pump at the bottom of the filter bucket and fashion a siphon-overflow from an old HOB
filter. Siphon the water from the tank into the HOB
overflow with a hole drilled from be bottom of the HOB
and a hose connecting it to the bucket. Gravity will pull water out of your tank and the retun pump will pump it back up. Put a ball valve on the hose from the HOB
to the bucket if you need to restrict the outflow to match the return pump.