I didn't want to dampen your dream pond but did want to point out the amount of water you'd have. I hear it many times
to go "bigger" when starting a new pond, but when the realization of having to
filter all that water, then cost starts
rearing it's ugly head.
I think when you get to a volume where it becomes financially impossible to filter all of it, then it
becomes time to think mud pond... but then clarity is sacrificed and often times a natural inflow of water is needed or some
way to add fresh water. You might look through the mud pond forum to see if you would rather forgo clarity for size.
With your new drawing I'm thinking the island is an important element in the project because even with the trouble it might
pose that people have pointed out, it still remains. So I think you're on the right track in downsizing the volume of water and
keep the island if that's something you really want to include. My lower pond is a "stream" sort of design which normally uses
a sloping bottom, but I've never noticed a difference by having a even depth. What kind of predators did you find visiting
your existing pond, if any? Maybe an even 3' or 4' depth might be enough to keep critters at bay and reduce the volume of
water?
Don't give up your vision! It took me 3 years to finish my first pond due to my budget... but as they say, if there's a will,
there's a way.