And I'm pretty sure I won't have to be afraid of the Unbidden bothering me at all.*
Uhm...
I think I was mistaken about that part.
Those stupid Awakened Empires don't do shit against the Unbidden, despite the fact that they are the only ones that COULD do anything.
In the end, they only defended their own planets, while the Unbidden could swallow up the entirety of the galaxy (except me, not sure why) and build up 500-1000k (who knows). They just annihilated my overlords and my by then rather nice 100k fleet made its suicidal run on the Unbidden portal.
As soon as I arrived, about 300k of Unbidden moved in. Naturally, my (well-balanced) fleet was annihilated quickly.
My initial idea once the Unbidden arrived was to make a suicide run on their portal before they can do anything. But since I was not independent, I could not make open border contracts with the empires in my way (and declaring war on both of them just to get through was out of the question as well). So they could swallow everything else before dealing with my overlords.
I don't really think I ever stood a chance in that situation.
This game needs a special crisis diplomacy VERY badly.
"Let's just forget our quarrels until those guys are dealt with" - Impossible to attack other empires, open borders, automatic aggression focus on Unbidden/crisis. Afterwards, restore old relation state plus some opinion and trust bonus.
It's entirely possible to defeat giant 200K doomstack forces with 30K. The key is to understand that 30K is more than enough to toast a spaceport. Build up mighty ground forces, create a distraction, toast the spaceport, and SPESS MEHREENS!
I don't see how that would be possible. Once in a system, my fleet would just attack their fleet and any other fleet would automatically help in that battle instead of going for the station. It's not like you could say "ignore incoming damage, just attack target X".
Plus,
MadMaxHellfire is fully right about the defense stations.
But, let's assume that was actually possible.
Then what?
I would've grilled one of their planets, but their other planets would have remained intact, just as their fleet which would be free to wreak havoc on whatever I have.
Maybe you had a Fallen Empire with just one planet?