It's pretty much the same design I did in post #16 so I'd still go with that if more BD's and filtration isn't
doable. I would still not do a BD to pump type design, but save up for at least one Ultra III sieve. I really think
a mechanical filter before the pumps is a must. But at the very least, if trying to do a BD > pump > s/g filter
design, add a drain on the bottom of each of the s/g filters to be able to release larger debris that gets trapped under
the s/g filter grates. I'd also go ahead and dig a filtration pit now so you could add filters and have them be at pond
level so the BD and skimmer plumbing could gravity feed them if you wanted (needed) to in the future.
I think there is a temptation to either split the return of a s/g filter to feed more than one thing or combine the output
of 2 s/g filters to save on plumbing as opposed to just running one return per filter to wherever the desired output will
be. I've seen a lot of s/g filters returns that don't work as anticipated because combining and/or splitting them up. I'd
just keep a constant "one filter, one return" type design. I think your design does this but thought I'd toss it out there
because sometimes people see multiple lines in one place and think, "why don't I just use a larger pipe and carry all
the water in one?", and sometimes it complicates things even more.
Will the island get some type of support around it? I've never done anything like it but all the other designs like this
that I've seen has the island surrounded with some type of support that I presume is to keep it from collapsing into
the pond, much like a collar does around the pond perimeter. Do you plan to do something similarly?
I'm not sure who the young person is in the earlier picture standing on the island, but she gives scale to the size of
the project and it's a huge undertaking!