So i have been thinking

I am designing a gravity system

Originally i wanted the pond to stick up 0.5m in order to accommodate the filtration at surface level, but the more i look at things the less practicle it seems .....

so i am considering a gravity system at ground level, but then i thought when i flush my mechanical filter how do i get rid of the water i am draining ????

then i thought if i dug a mini septic tank to the side and below this filter box do you think this is a viable way of draining the water????

I would just pump it out with a pool pump.

You can use your existing pump(unless you have submersibles in which case you can add a pressure pump for this purpose) by creating an extra line with valves and pump it to your normal drain.

I think it will be cheaper and easier to install a pump for this purpose than to build a septic tank/french drain only for this.