It is possible to brush paint large areas successfully, but it requires patience and practice. Airbrushes and compressors have become much more affordable. Airbrushing requires patience and practice, too, but the learning curve is much steeper.
No, this technique does not make the paint too thin. You are just dipping the brush very quickly in the thinner. I recommend that you try this the first time using Tamiya's thinner. If not, add a tiny amount of acrylic retarder to the thinner if you can.
The other thing with Tamiya is to keep the brush wet and lay the paint on relatively heavily. You want just less than will cause a run. Takes time and practice.
Good luck!