Well, this is first year with so deep water, so i dont know yet about the temp at the bottom.
Today, it was 6,4 celsius at 3 meter deep. Thats 2 degees more than in my old pond that was 1,5 meter deep.
Normal winter can bring airtemp down to -20 C. or more.
There wil be some heating system when the pond is finish, that hopfully will give 8-10 degrees at the bottom, winter time.
My Koi lives outside hole year, and it has worked ok for 7-8 years. In a lot smaller pond.
It freezes totally over, with 30-50 cm thick ice.