If you had 300 battleships and lost 700 ships, then it wasn't just the planets defenses that killed you, they must have had stationary defenses or their own fleet as well. I've captured homeworlds with a mixed force of about 500 cruisers and battlecruisers, much less battleships. And like ChickenHawk said, a battleship force to kill an otherwise undefended colony takes very little time to build (in terms of a 2 month game). A couple of days is nothing.

On the other hand, I do agree that the game seems unfinished in a lot of regards. Mainly in terms of visibility of information (trying to figure out where the heck you have harvesting/trade routes set up), the sheer number of steps it takes to actually DO anything, the inability to do things like automatically create a fleet from all ships of the same class in a system instead of having to fleet one, go into fleet view, add the rest, inability to give orders from fleet view, etc...