I do use a combo of different paints including humbrol enamels. The Humbrol paints do need a fair amount of thinner to spray and not clog your brush. You are right in saying they are not airbrush ready.
The thinner amount varies a little from can to can. Each have their own consistency. Eg for yellow I added 5 mm of thinner orange more like 6-7. I use the humbrol branded enamel thinner.
For thinning and mixing I’d suggest getting something like tamiya mixing jars. Carefully pour your paint in, thin and mix thoroughly. The paint just bonds to the plastic pipettes and kills them. Same with syringes.