We have likely all had to just go through the experimental stages of learning what works, and what doesn't. Speaking strictly from my own experience, this works for me, and I am a Tamiya acrylic user.
First, a thorough cleaning of the airbrush. Then make sure you have completely stirred your paint, just shaking the bottle is not enough. For thinning I use both X-20A or 71 to 90 percent alcohol, I don't notice a significant difference.
I use a 50/50 mix to start, that is for broadcast work, like a full coverage base coat. I make adjustments as determined by test spray results. The majority of my work is at a psi of 15 to 20. I use Badger gravity feed airbrushes.
I know it sounds contrary to the general method for thinning, but this works for me. I will often end with a thinning ratio of 60/75 percent thinner to paint, for smaller areas like gear wells, cockpits and such. It is watery, but sprays very well, just be careful to avoid runs. With practice it is amazingly easy to control and produces some really nice, smooth coats. And remember, the thinner the paint, the more you have to reduce the paint flow. You can't have a wide open blast when paint is thinned more.
Try a new bottle, THOROUGHLY stir the contents to a smooth blend, transfer to a small container and add thinner, get some scrap plastic and start just varying thinning ratios, psi and distance to the surface. In general about one or two inches will be enough to prevent the rough, pebbly finish as shown in your example.
I'll be surprised if you don't quickly find finishes that you like, once you've tested enough to learn what works best for you. Good luck, let us know how it goes if you find time.
Patrick