Welcome donb :wave:
Southern mid-west here
Our temps are running 27* in the morning and 52-57* afternoon temps. Not quite time to turn the pumps off yet, but getting close. Fish are still somewhat active in the afternoon, really don't see them till then. I have a submersible in my small pond, when I shut that down I'll pull it pup clean the filter material good and store it dry in the garage. Then put the pump back in the bottom of the pond for the winter. Usually submersible pumps should stay submersed even if you pull it out to store it, so it's just easier to leave it.
The bigger pond has an up flow above ground filter, that I will likewise drain and clean well, and store dry in the garage. Any tubing will go to the bottom of the pond and will unhook and drain the pipes to the waterfall. I do not use aerators, as we normally do not freeze for weeks at a time, a week of frozen pond I'll not worry too much, but anything over that I'll break out the pond heater and get a hole open. Word is, we're in for a unseasonably colder winter this year, so I might just throw the heater in the large pond, just incase it's needed. The small pond is up against a southwest corner of the house and has never frozen over solid, protected by the house and trees I suppose. Anyway, most of us close our ponds for the winter, minus a few in warmer climates,
as the good bacteria in your bio filter will not survive in freezing temps, and will have to regenerate in the spring.
Mainly just be sure to clean your filter material well and store it, and do not let anything above freezing level, sit with water in them, including pipes and tubing, and your fish should be fine
5 ft deep is a good depth for koi and goldfish do fine in shallower water. Good luck! And welcome!