I think those are perfect for a tabletop force. I would happily both game and display those.
While a dark wash may draw the colours together, it will also darken/mute the entire mini a lot, as they are already quite gloomy, so quite some work will be required after that.
Why not leave as is for now, and maybe expand the force with new models from the box where you take an different approach to painting them? If your results get much better, then when you have finished the force, you can consider what to do with the old ones (improve paintjob, strip and do over, leave as be).
Or you could try applying subtle hightlights to edges/raised areas that has not gotten them yet? Especially some of the metal bits are a bit dull as is.