We live in Jackson, MS, but used to live in Cambridge, UK. I am
wondering how much insulating would actually help your ponding
experience.

Our pond here seldom has more than a few days with ice on the top.
Our external temp is generally in the upper-30's to upper 40's in the
winter. We have not insulated or covered our cement pond at all.
That means the temp does go down to air temp quite fast. The koi do
fine, but are pretty inactive when the temp gets really down. We
don't feed them once the water reaches 50 degrees.

I am not sure how insulation would benefit them or us if it did not
keep the water high enough to feed them. Even then, I suspect it
would reduce the amount of time under 50 rather than eliminate it.
Jan may have some wisdom about how we would benefit if we insulated.

I think you will love your larger pond. People here might have a lot
of suggestions about how to reduce the amount of maintaining work to
be done.

By all means post some pics as your work moves ahead.

Jim