The Pond Digger is a great resource, as are the videos by Aquascape. Both have a ton of great videos that show you lots and lots of pond that are built in the eco-pond style where koi thrive beautifully.
The biggest piece of advice I would give anyone who is keeping koi in an eco-pond is to manage your fish load. If your measurements are accurate, you have somewhere between 7 and 8000 gallons which is great for the number of fish you have right now. Koi get really large really fast! They are fun to watch grow.
Stay off the dedicated koi pond forums - they are extremely knowledgable about the style ponds they own (basically swimming pools for koi), but they will scare you half to death about a rocked pond. I hung out on a few the first few years I had my pond (until they kicked me off - haha!) and tried to add balance to the conversation about keeping koi in garden ponds. They all just kept repeating "Your koi WILL die. They may not be dead yet, but they WILL DIE." They would warn me about the toxic build up that my rocks and gravel would cause that would eventually be the end of my fish, and apparently me, too, if I dared dip a toe in the water. Aquascape was a literal swear word on some of those sites - you couldn't even post the name. And to be honest, you may even hear that here - not as much as you would have a few years ago, and not with the same vitriol, but there are still some DKP people here who will tell you to get the rocks out of your pond if you hope to be successful. But there are also lots of people who are willing to look at the results that others have and concede that eco-ponds are an excellent home to koi AND goldfish. To anyone who would say "get the rocks out" I would say "get the plants IN" - because an eco-pond isn't balanced without plants.
The one thing I question is your pump size - if your pond is as large as you believe it is, your pump seems undersized. Your goal should be to turn over the water 1 to 1.5 times per hour. A 6000 GPH pump sounds close, but it would be right at the lowest end of what would be recommended. I would talk to the guy who built your pond to find out how he chose the pump size.
The one argument I have with Aquascape-style pond builders is the clean outs they recommend you do to your pond. You'll see videos where they completely empty the pond of fish and water and hosing down or even power-washing the rocks. Some people want that "new pond" look every year and pond builders like the revenue stream. I would never strip a pond down like that - you end up with "new pond syndrome" year after year. Let your pond develop that lovely algae and biofilm that adds so much to the balance of the pond. If you have some reading time, search for Meyer Jordan here on the GPF - Meyer was an extremely knowledgable pond builder and pond keeper who, sadly passed away last December. But he was a regular contributor to this forum and he left a wealth of information and advice here that you can read.
Hang in there! There's not a single first time pond owner who hasn't had moments of being nervous about whether or not their pond was going to function properly, or if they are doing all the right things, or if their fish all look OK. If all goes well, you'll be back here in a few weeks posting "DO I HAVE A LEAK??" Haha!
My pond is a hybrid, not a dedicated koi pond or an eco pond and it can be a lot of work...but I still love it 
