I'm normally a proponent of leaving things alone, but in this case it doesn't look like there is much to disturb and it would be nice to see what's going on anyway. I would drain it. It would be nice if there were a big pile of sludge that once removed would increase the depth. It can get very hot where I am in 7b and fish might not survive in less than a foot of water and would likely freeze solid under some winter conditions. If depth cannot be increased, I suspect fish would be a very bad idea.

Yes, a bunch of potted plants in the shallow water, a bunch of acquarium under water plants tossed in the deeper water (don't collect them and carry them to warm mediteranian climes to contaminate the whole sea, they grow super well even in zone 7b so they are verboten in natural environments), and nice plants on the edges that soften the edge would look nice, give shade, and create a hopefully good pond environment. Water flow would give aeration and help plants filter the water. However, again, the low volume might be an issue from over heating or cooling that could be increased by flow.

I havr feeder gold fish in a kiddie pool less the 2ft deep . As lobg as you dobt get hars freezes they should be fine. Ans they are cheap to buy !!