I would suggest using some of the soil from the dig to make a berm all around the perimeter so the level of the pond will be above the surrounding terrain. This will avoid rainwater washing into the pond and bringing unwanted substances to pollute the water, minimize issues with floods, avoid groundwater seeping under the liner and also make it a little easier with gravity filtration. My pond is roughly the size you are planning (16.5'x26.3') and maybe I should have made it longer and narrower, more similar to the shape is now favoured by the top Japanese koi farms, something like 35'x12'. If your climate is not too extreme 4'-5' deep should be all that is needed. In my opinion when planning a pond it is best to get right the things that can't be easily changed later. So size,shape and build method of the pond, bottom drains and underground plumbing. Also plan for valves in strategic locations to cut off the flow if maintenance is required and a means to purge the bottom drains if sludge has built up. If you have strong prevailing winds in your area, put the skimmers where they are more likely to push the fallen leaves and place the returns to work the same way. Ciao
46000 liters with only wetland filtration