Looking at your plants, I think you have iris, hyacinth, water lettuce, some type of lily, and some others I don't know. If the lily is a hardy one, that and the iris could probably stay outside. The lettuce and hyacinth need to come in if your temps get down to 60 or below, also the lily if it's a tropical variety.
As for the pond itself, an aquarium heater would probably keep that from freezing. I'm sure it would if you place some bales of straw around the enclosure and stuff more straw in the enclosure all around the tank and run your pump to keep the water moving. Looking at it, I think you'd need like 8 bales of straw, but it's not expensive and easy to find at livestock feed stores in the fall as people use them for decoration for Halloween. Make sure it's straw, not hay! 2" Foamboard/pinkboard insulation would also work, but I'm not sure you could wedge it in there unless you took the top railing off the enclosure. Straw you can stuff in and pack it in from the corners where you have more access. I use this heater:
https://www.amazon.com/gp/product/B07JDQQ51Q/ref=ppx_yo_dt_b_asin_title_o01_s00, keeping my pond covered (last year I used a storm window, this year I'm building a cover made with a clear roofing panel) and pump running with a fountain head that adds oxygen just above the surface, and it doesn't freeze in 5a. I feel like a pond heater would be overkill for a container that size, and pond heaters are often very high wattage. For much lower wattage, you'll get much better results with an aquarium heater for a pond that size. I turn off my waterfall in the winter and drain the hose, to prevent freezing and overflows caused by them. I would just remove yours completely since it looks pretty easy to do and will make covering the pond much easier. The fountain you have in one of your build photos is perfect for winter, just use a different head as the aquabell will freeze. This kit has a great oxygen-introducing head for winter (the head on the far left in the product photo):
https://www.amazon.com/gp/product/B004DL0Y44/ref=ppx_yo_dt_b_asin_title_o02_s01. The heater can just lay on top of the filter box (put a flat rock or a brick on top of it to hold it in place). I have not had any problem with it melting the filter box on mine, and that ensures it stays away from the sides of the pond, which would be a bigger problem if it got melted, and that the water going into the pump is at its warmest. If you go this route, the fish can for sure stay outside over the winter, as the water should not freeze over at all in 7a. You could probably run the heater at its lowest setting and be fine in zone 7. I do have to crank mine up to the highest setting in the dead of winter when it's hovering around 10 degrees for over a month.
