Is the example image (https://github.com/worldveil/photomosaic/blob/master/media/r...) with or without opacity cheat?
Basically what this will do is assign each tile less than 100% of the time randomly and then for each that isn’t, you choose among the top best K matches on L2 distance with equal probability. Gives it a little bit of both.
I’ve found that opacity if 0.7 is often a nice compromise.