Welcome and thanks for joining Koiphen!
Long initial posts can be hard to know where to start when responding, so I'll just kind post some things
that come to mind, not by importance or anything.
The dimensions show to be a little over 7000 gallons which is pretty good size imo.
If the pond is for koi I'd nix the shelf idea. The PNW is known for herons and raccoons that will use the
shelf for a feeding platform. Straight sides down to the pond floor will discourage them from wading.
For BD's a general guideline is an aerated BD of 4" will be effective for about 12' diameter flowing 3500-4500 gph.
A 3" aerated BD flowing 2500-3500 will work for about 10' diameter. You can start with that to estimate the number
to use depending on the dimension of the pond floor. Ultimately it will also depend on the number of gph your filters
can handle and how many egress points you have to the filters. On a new pond I'd personally stick with regular BD's
and not have to look at retro BD pipe in the pond. A retro drain works best going through the side of the pond so
it still requires a penetration. Regular BD's installed correctly won't leak. I bet if we did a poll of everyone here you'd
see a very small number that ever had a BD leak, if any at all.
Many use pond lighting from over the water as opposed to in it. It's easier to install and doesn't show debris in the
water nearly as well.
There are several good style of auto fills and having the one you'll use before building will allow you to install and
hide it well during construction as opposed to trying to add it later. Here's one:
https://www.jandy.com/en/products/water-leveling/k1100
Pond design is almost limitless... search the forum and you'll find all different kinds. Here is a thread that shows off
some of member's ponds:
https://www.koiphen.com/forums/showt...-Koi-and-Ponds
Waterfalls:
https://www.koiphen.com/forums/showt...-photos-PLEASE
YouTube videos:
https://www.koiphen.com/forums/showt...ers-on-YouTube
Looks like a great project! Go slow... read as much as you can.... ask lots of questions... and have fun!