If you wish to stick with enamels, Model Master do a good range known for accuracy (though, having largely converted to acrylic, I’ve not tried them – I just used to work in a model shop. Vallejo is definitely my favorite acrylic range, though I also use mixed media.

I can’t remember it’s availability in enamel, but it’s probably worth investing in mediums (numerous types are available, for different purposes, but the ones to which I refer are basically paint without the pigment). These will act in the same way as thinners, but will maintain the consistency of the paint, if that makes sense. Flow enhancers are a good example, achieving a smoother finish without requiring several extra layers.

Such a thing would also be ideal for the thinner paints you mention, thickening it without affecting the colour.

You will, of course, have to check such things are available for enamels (I seem to recall Humbrol doing so, but I’m probably wrong as I really can’t remember).

Mark.