I'd really like it if there was somewhere that the number of planets you had left to colonize at your particular level was listed. I tend to not colonize all the planets that are available to me because I don't know how many I have left to colonize, and don't want to make the mistake of colonizing one that I don't really need.

Sure, makes sense to see it quickly on planets page. Although it's easy enough to calc: rank/4+1