Even if you use 1280 x 720 resolution, the black bar will remain and the image will not be centered. On a tablet with a 16:9 aspect ratio there is no such problem. I tried the same resolution in a similar application SuperDisplay, there is no black bar.
for clarifications, could you tell us what is the native resolution of your Android device?
you can check this by going to viewer settings -> check “Use native android resolution” and uncheck the custom resolution -> connect device to spacedesk -> extend -> go to windows settings then check the resolution of the client.
please refer to the attached screenshot for assistance.