the mechanics of sending colony ships to colonizable planets is pretty bad.

In another space 4X game, they had it done really well -- you could do it one of two ways --

There was a screen that displayed all known planets.  That display had a toggle for whether to only display the colonizable ones.  It also showed you how many idle colony ships were floating around your empire, and which planets already had colony ships en route.  You could sort the list by planet size/class/etc., and just say "send a colony ship" to those desirable planets without one already en route.    The nearest one (I would argue to calculate "nearest" as "fewest turns away") would be dispatched automatically.

Also, if you were inspecting an individual ship, you could give it orders to "colonize", and be presented with the aforementioned list of colonizable planets, sortable by distance/class/etc., and choose to send it to the nearest or most desirable planet that did not already have a ship en route.

Right now I go cross-eyed trying to remember which planets were which and which ones I'd intended to colonize with which ships, etc.  The above would help a *lot*.