I know you shouldn't release them into the wild.
Otherwise, not being a koi keeper I don't think I can help.
I hope someone else can help.

Why don't you set up your small pond without fish and as a wildlife pond. You should get a greater variety of pond life if you do.

Good idea with the small wildlife pond. Decorative fish are great, but wildlife these days needs all the help it can get, and garden wildlife ponds make such a difference.

And the beauty of a wildlife pond is breathtaking!

Chip

...and you don't have to feed the fish, filter or test water etc. Or get too upset when herons call.