I seem to be one of the relatively few who handbrushes Tamiya acrylics without difficulty—why, I don't know.
The best solvent for Tamiya acrylics is either their proprietary thinner or 90 percent isopropyl alcohol. If you use the latter, adding a tiny amount of acrylic retarder is a good idea. Tamiya's thinner has been reported to be 90% isopropyl alcohol with a retarder already incorporated.
For extensive hand brushing, I definitely recommend using Tamiya's thinner. It saves time an hassle. And Tamiya paint does benefit from very slight thinning for extensive brush painting. Because the solvent system in Tamiya acrylics is alcohol based, they tend to lose solvent fairly rapidly at the surface, causing a skinning effect with the thicker coating resulting from brush application. Adding a small amount of thinner, especially with retarder, compensates for the solvent loss and makes painting much easier. You should also always paint with a "wet edge" and never go over a recently painted but uncured area. Wait until it cures and give it another coat.
Brush painting produces a much thicker film than airbrushing, so cure times are extended. For a large, brush painted area, wait a minimum of 36 hours before recoating, and 48 is much better.
As for the amount of thinning, that's largely a matter of learning from experience, and the amount will depend on the age of the paint and how well it has been kept. I have previously opened bottles of Tamiya acrylics dating from 1982, and they are just fine. They've lost a little solvent, but that is easily replaced. Replace lost solvent only with 90 percent isopropyl alcohol. Do not use Tamiya's thinner to replace lost solvent.
For small jobs, I dip the brush in isopropyl alcohol before dipping it into the paint, and repeat this every time I reload the brush. Never paint straight from the bottle unless it will be a "one brush load" job. Decant paint into a smaller, separate container, trying to decant just slightly more than you think you'll need.
Everything gets easier with practice and experience.