If all you will have is plants, you could get by without a pump/filter. Can you take a few pics of where your pond is now, to give us an idea what you are talking about.
I am surprised that you were able to find a pond kit (your 1st pond) that was for 6 feet deep, AND only 8’x8’ wide! Most of us don’t go that deep, as 2’ - 3’ are sort of average pond depths. Gosh, I’ve dug a lot of holes in my life, but never anything 6’ deep, and even my pond that is around 3.5’, we used an excavator to dig. It’s no wonder you had trouble with that particular pond!