With a 50% water change you will have the same readings unless you have ammonia or nitrite in your tap water. The nitrates should read 20 ppm
after a 50% water change assuming that you don't have any of those in your water either. I'd do another 50% water change as 20 ppm
is still rather high. It's on the upper limit of the "acceptable range" and you should try to get it as low as possible, definitely under 20 ppm
It could be a swim bladder issue or it could be normal behavior for your fish. I too have an angel that has done the same thing for 6 months or more and it is perfectly healthy. It always uprights itself when it is time to eat but I often catch it peering up as you described.