Suggestions for some of the orbital units.

Yes I know, but your comment made me remember that other thing.

Maybe they can just have ballistic arcs? Like to discover other stars/planets/factions you have to build telescopes to find them and when you get into space with satellites and space stations you can build super expensive invasion craft that can fly to other systems. In reality, you have spacecraft take ballistic arcs using gravity wells to accelerate out of the system. So make them use a gravity well to accelerate and maneuver then when they are out in the void they either can't maneuver or have very limited ability to alter their trajectory. Maybe they only have a set amount of power/fuel and have to recharge around a star or refuel somewhere in a system you control.
So maybe the attack/invasion ships are expensive and huge with massive fire power and armor/shields to crack a planets defenses then they release waves of smaller attack craft and landers to take the planet/system. After you land and set up a staging area you can build a teleportation portal or something to get reinforcements and heavier units in system. The defenders could build planetary defenses to destroy an invaders home ship and stall the invasion and counter attack with their own ships. I feel this approach would make a system invasion a multi-tiered event and an expensive and risky proposition but potentially massively rewarding if the system can be held. It would give that epic feel that most RTS games seem to be missing.
What do you guys think? Are there any points I have missed or are there any major problems with this idea?

I think your version of PA is beyond the scope of what Uber is envisioning.

Still, I think your ideas are creative and I'd like to give it a try if you decide to build a balance mod to do this.

Why did I just get a flash of "clamp on space thrusters" for planes?