I haven't tried oils so far, because I'm sort of intimidated by all the solvents and all the complications. I guess the long drying times are an advantage for mixing on canvas though.

I think part of the reason my mixing space ends up such a mess is that I need to add lots of water before I get the consistency I like (I usually paint many thin layers over each other). So I may mix a color in one of those little dips on the palette, even trying not to use too much color, but then when I add water it is still is to opaque, even though the dip is filled, so I transfer a bit to a new dip to make it thinner again, and run out of room soon. Mixing on the palette itself works okay for me in the last steps of coloring when I use thickest paint.

I have now bought some of the larger, but stackable mixing dishes astridv recommended above, and will give these a try. Maybe those are more suited for my watery approach.