There's a couple things you can do. First, you can use lacquer thinner to reduce Tamiya bottled acrylics instead of X20. Generic thinner will work but I like Tamiya's own lacquer thinner or Mr Color. They're both pretty mild and have never damaged any plastic I've used them on.
However, you may not want to use lacquer thinner. Completely understandable. If that's the case then try alcohol. I prefer denatured alcohol but iso alcohol will work as well. If you use say, 70% or 72% alcohol, it's almost like adding retarder or a leveling agent. Not exact but almost. 90% alcohol, not really. And denatured, no. But, you can thin your paint much more with 90% alcohol and even more with denatured. With denatured, almost to the point that you can fill a bottle with alcohol, add enough paint to color it, and spray it. And surprisingly still get very good coverage.
With that said, I've found that even with 70 or 72% alcohol I could thin more than with X20. With higher percentage or denatured, a lot more. And close to that with lacquer thinner. I'm generally referring to flat paints, I understand that denatured and 90% don't work as well with their gloss paints but I haven't used them often. Lacquer thinner will work just fine though. Either way, tip dry will be all but non-existent.
One other suggestion is to pour some thinner, alcohol or lacquer, into a separate jar or bottle and mix your retarder with it. If you choose to use it. That gives you more control of the mix.
If you're in the US then I know denatured alcohol is pretty inexpensive. I think it's still between $6-7 a quart at Wal-Mart. Tamiya lacquer thinner or Mr Color would be higher. Give denatured a try. It doesn't smell and it will work. I've run a lot of it through a fine needle Sotar.
Tony