A good reason to take your time and solve as much as possible in the sketch/thumbnail phase is that when you go to do the real thing it's a 'paint by numbers' situation where you only have to worry about rendering. Having all major decisions decided before you commit saves time.
Would anyone be interested in buying prints of any of these pieces? Or is there any piece I've done (non-commission) that you'd like to be able to buy as a print?