Back in the polluting good old days, there used to be a lot of enamels and lacquer paints that could be easily painted by brush and could be sanded/blended/feathered. These days in the world of water based paints and powder coated paints, it has become very difficult to paint by brush and achieve a finish that looks good.
I recommend you look up some automotive factory touch-up paint companies and ask them what they recommend these days. The last touch up painting I did was about 10 years ago using an air brush. Air brushes are inexpensive relative to the cost of paint these days. (Aliexpress probably sells them too.)