I had to sign up to comment. Sorry I'm sort of skipping the introductions and jumping in. Hope nobody minds. I was going to post this an a new thread but instead I wanted to pick the brain of the OP. Hope you are still around and listening.
I did a design similar to yours but for my 500 gal above ground pond. It has worked great for years and with as close to 0 maintenance as I could imagine.
I like to follow the design concepts of :
1. No holes in the liner.
2. Gravity feed to your prefilter so your pump doesn't grind up the fish waste making it harder to filter and feeding the algae.
3. Gravity feed to your settling chamber so you can filter before the pump. (can be the same a pre)
4. Biofilter after your pump so the biofilter never needs cleaning.
5. Always biofilter in a dark oxygen rich environment like an enclosed trickle tower spraybar setup into your bio media.
6. Waterfall back into the pond for oxygen
The only way I know to do this is with a siphon tube.
It works great on my 500 gal above ground pond but it hasn't translated that well to my Mother-in-laws 1500 gal in ground pond.
In my pond the siphon is a U shaped tube where with the pond filled and prefilter 1/3 full I dunk the entire U tube in my pond to fill it underwater. Put caps on the ends Pull it out and put 1 end in the pond and another in the pre where the end is underwater. I pull the caps and the prefilter fills in seconds to equalize levels and I know I got a good siphon that can last all year sometimes.
Doing this on a big pond and at ground level does not work well. Its such a pain I've been contemplating cutting a hole in the liner. Maybe just a foot below water level in case of a leak it wont be catastrophic. From there I can run the pipe inside the pond to the bottom area and plumb to 1 foot down into the side of my prefilter.
Just as I was thinking this was a good compromise my mother in law left the water running and overflowed the pond. A couple koi died but we saved most of them. But what really worried me is my buried trashcan prefilter was lifted out of the ground when she overflowed her pond. If I had connected my pond to my pre like I was planning I certainly would have ripped the liner as the filter rising would have lifted my pipes up also.
So to the OP,
@Ray G, how do you get your siphon tube full of water while keeping it in place? I was considering a couple of ball valves just below the water lines. Is that what you did? You cant just open the inspection plate and pour water in. Whats your process?
Thanks for the help.