I think it's very important to make attacking/conquering planets way more difficult than defending/keeping them. It's not much fun if colonies are conquered or destroyed all the time, on both sides.
Imagine a war where both opposing empires each just send a huge fleet into enemy territory to destroy or conquer everything. The only way to end this is to withdraw your own task force and defend your planets, which is difficult since you have to catch your enemies' fleet first. Even if you succeed, your enemy will probably have destroyed most of your empire by that time.
There should be strong system defenses that can do lots of damage to an attacking fleet before they are destroyed. For example very long range beam/missile bases which can take out a few ships before they are in weapon range. This way, it will be very difficult to conquer many colonies in a row.
If ground based defenses can hold off an armada of starships, it will only promote turtling and make the end game that much more painful.
But you want some solid defense in early game, so miltary/industrial races can't just overrun the others empires.
I don't want ground defenses to be able to hold off a huge enemy fleet. I just want them to cause considerable losses to the attacker, so destroying/conquering a planet comes at a price.