Cheerios, LOL! Never thought of that....nor hand feeding, but I always stood around while they were dining.
Don’t think I can be of much help on this, but FWIW:
My koi only liked Tetra Pond Floating Koi (and/or Vibrance) Sticks....period. Back of box has a feeding/winterizing schedule. I tried the flakes one year (no go). IIRC, they had a different nutritional composition. Even tried Tetra sticks with a different nutritional composition. No go.
I started to wing it. As the cold weather approached, I started feeding them less and observed how fast they ate the food. When I shut off the waterfall and winterized the pond, I continued to watch their behavior, their sluggishness as they became more dormant and how they responded to food.
I never took the water temp. One advantage that I had that you may not have is that my pond freezes solid every year & stays solid throughout the winter. Don’t know winters in northern VA, but they may fluctuate more wildly than more northern climes.
I bought my koi from a well-respected local nursery, where I also bought most of my trees, shrubs, flowers, etc, etc. I quickly learned who was the most educated to answer my questions on what issues. Usually the managers (but not always) of their respective divisions.
If I were living in northern VA, I would do the same to start off with-—even more than expert advice from the net. Local nurseries like happy regular customers and usually offer excellent advice.
The Federal Government is an entity that makes a mistake, and then repeats it over and over again until the mistake becomes THE LAW OF THE LAND.
BTW I get absolutely everything for the pond on Amazon now. From food to chemicals to landscape lights to pumps, it's MORE than half the price or less from my nursery.
Also I'm trying something new for the Blue Heron, a mirror ball that floats around the pond on the current from the waterfall. The reflection is supposed to scare them off. I'll let you know how it works.
As far as freezing, my pond guy says we get 7-18 inches depending on the year. But since my pond is old and built out of concrete he recommended I use a De-Icer so it doesn't crack the bottom. It also gives a place for my birdies to drink in the winter. Got a cheap 1,500 watt one on Amazon.