Here are some thoughts for what they are worth.

If mechanical filtration means some sort of filter pad, there are
somewhat easier ways, although there are good 55 gal drum set ups that
have pads. We have tried to avoid them.

Veggie filters are wonderful (shallow ponds with lots of plants
through which pond water flows at a sloish rate. 10% or more the
surface area of the pond.). They provide lots of roots to do
mechanical filtration and they have lots of roots for bacterial
surface and they have lots of roots to grab nutrients and convert them
to plants that you can yank from the pond. We have 2" drains in the
bottom of the berm ponds, We drain them out onto the lawn annually.
Great fertilizer.

Our pond is shown on our bellsouth web page: home.bellsouth.net/
personalpages/pwp-jameshurley The berm ponds are the filters.

The berm filters do all the work. The pump takes water from the low
part of the pond and runs it into the veggie filters. As it slowly
passes through, the muck settles, the roots filter the particles and
the nutrients are grabbed for plant growth.

Annually, we too use a pool net to take out the stuff that does not go
out of the pond to the filter. The intake for the pump is in a 5 gal
bucket with loads of 1/2" holes. Anything over the 1/2" does not get
sucked in. We net it out from the low point. Easy to do.

The bottom of the pond is bare. Plants in the pond are in containers,
except for some escapee lilies. The bare bottom lets the koi 'sweep'
the pond when they swim by. The muck they stir up goes eventually to
the deepest part of the pond.

Others will have good ideas for you.

Jim