I'm late replying, sorry.

That's okay

For the filter, if you can gently slope the bottom to the center or one
side/end, you can have the muck collect and be easier to vacuum.

I can do this.

I wonder how it would work if in addition to the above suggestion, if
I used something like a GIANT colander for the water to run into right
at the start, and then it would filter through and down...think the
muck would collect in that first?

Have decided to use a pre-formed pond (shallower than my original
planned trough) I have nothing in, for the filter, so will be working
on that this upcoming 3 day weekend...

Let us know how it works.

Will do!