I'm not a fan of pump fed filters as the pump impellar pulverises dirt into finer particles which are harder to remove and increases maintenance frequency.
You could "suck" from those depths/areas into filter instead (gravity fed) with one or two pumps after the filtration. This also hides pumps from view and stops some inquisitive fish getting stuck or chewing species damaging power cords and sponges etc. But you may have other reasons to keep this configuration. As long as you know what you want.
With the bottom drain, there are some things to be aware of if this is your first one.
- Make sure the bottom pipe drain doesnt have any humps/hills. Air pockets will collect ruining water flow and are difficult to remove.
- Add a purge tap if possible. And an external clean out - screwed cover.
- Put a wide cover plate slightly raised over the bottom drain to improve dirt suction and stop bigger fish.
- The pipe diameter should be chosen by water flow rate. A wider diameter needs more flow otherwise sedimentation inside the pipe will occur. Sometimes 2 smaller bottom drains are better - sometimes one large drain is best. Tank floor area, tank shape, tangential returns, air diffusers etc will affect your filtration design.
The water depth of your tank lends nicely towards air lifts if you want to move lots of water volume cheaply. A 100mm (4 inch) diameter pipe thats 2.2 meter high can achieve 16,000 LPH (4000 Gallon per hour) with a 40 watt air pump with the added benefit of boosting dissolved oxygen and cooling in hot summers.
Filtration is a large topic. Build in redundancy. Keep spare parts. For a bigger tank, build a quarrantine tank too.