I've done longer YouTube creative Styles at this point, with 9 by 16 aspect ratio. What aspect ratio do you typically use when testing, and do you test the others later? Also, when testing, do you start with images, short videos, or long videos?

Honestly, all that matters is making money. If you're making $300 profit a day right now, I would just scale what you have. That's my biggest piece of advice. You're doing 9 by 16 ratio, which is more native to Facebook. You can run and scale horizontal, square, and 9 by 16 ratios. If you were to test another one, test square (1:1). When you start, test a range of images, short videos, and long videos to identify what performs best. Longer videos have more psychology and do more to pre-sell and qualify the person, but images can be tested faster and still perform well.