Re: ship/planet/troop combat
Borrowing from an old computer game (PLATO empire, it's on the wikipedia): a player controlled ship could bombard a planet while in orbit, killing some of the ground forces but taking damage. After several bombings and damage taking the ship would break orbit, spend time repairing, then go back to the planet and resume bombing. It was time consuming, but you could whittle the planet's troops down to a minimal value ("remaining troops are too well dug in to be destroyed"); and hope you weren't attacked by another player's ship in the meantime (or have friendly ships guarding you). Then you can drop your own troops (there were limitations on how many you can carry) with an exchange rate of 1:1 and take over the planet.
I've written up some rules to do the same for a campaign, as yet untested, with changes to adapt to Starmada. In short, a ship can bombard a planet at range 1 for one round of combat per strategic movement/combat phase if no enemy ships present, firing every weapon (arcs don't matter in this case). Each damage point destroys 1 troop. The planet's defensive systems fire back like an Anti-fighter battery, the number of AFBs depend on the number of troops on the planet at the start of the bombardment ( for now 1 per 500 troops, may be too few AFBs, but...); at EVERY ship that bombards the planet that strategic phase (cuts down on deciding who they should fire at.. trying to keep this simple and quick). The AFBs probably won't really hurt ships much, but repair is not that easy to do, and ships may be damaged from recent ship combat. Testing would be the only way to adjust the defensive fire so it's hurtful but not extremely so. I have a limit on how many troops can be on a planet (2500 max for good production planets, 1000 max for not-so good prodution planets).
Next phase would be transferring troops to the planet to do ground combat, exchange range a simple 1:1.