Ammonia at 2 ppm will always be harmful, no matter the pH. It won't be quite as harmful at a lower pH, but it still would not be good.
You have several large koi in a relatively small amount of water. They produce a huge amount of waste and ammonia. That's always a recipe for disaster unless you have a tremendous amount of filtration.
I know you have added a small trickle filter, bit it is obviously not enough. You need to increase your filtration by a lot. I'm a firm believer that it's easy to have too little filtration, but you can never have too much.
Since you are planning on building a pond, I would go ahead and get a system for the future pond and use it on this pool. Be aware that the recommendations for filters are normally at least half what you need.
Those ratings are often based on a pond without fish, so you need to get a larger system than recommended for the size of the pond and for those fish. My pond has around 5 to 6 times the filtration recommended.
Don't base what you need by the number of gallons, but by the amount of waste being produced. And you are much better off with too much than too little.
