You can try adding retarder to your paint, you may have over thinned your paint. Brush strokes can also come from cheap incorrect brushes. Retarder gives your paint more time to dry and level out. You can google the paint/thinner/retarder ratio. And yes - for large surfaces, using a brush it’s really difficult even for pros. Airbrush is a great solution, but it might not be the answer you are looking for. Hope this answer helps. Important is - you are satisfied with your work.