He's looking good. Sitting up at the top isn't necessarily a bad sign, as long as he can go down if he wants. Just as long as he's not "gasping" at the surface. Most people say bettas are top level swimmers and like to hang out at the surface so he may be acting perfectly normal.
You can leave the tannins in as long as he needs. Like the epsom salt, they won't hurt him. He's looking good. I'm really glad he's swimming more. I love his big "dumbo ears." And I really love your sketch. It has a lovely simple zen look to it. You should frame it with a black mat and a silver frame. It would look stunning.