I'm not a fan of the colony invasions in their current state (in build 2.31.4 of Crusade).
I like the basic concept of Legions. They are expensive to build, and choosing to garrison them on a colony is a non-trivial investment, at least until later in the game. This all works for me.
What I don't like is the "mini-game" (best way I can think of to describe it) when you invade an opponent's colony that has legions, and you have to place all your legion units on tiles, and then click to start the battle. That feature adds nothing to the game for me. I haven't seen anyone on the forums say that they like it.
Would you consider just removing the invasion mini-game from Crusade and replace it with a simplified numbers-based resolution of the invasion? This would be one less feature your development team has to deal with, and hopefully will not be missed by many of the gamers.