Sacred Haven of Debauchery

That's the only direct question, Mente.

Obviously, it's cavalry with sabers; I said that from the beginning. Well, maybe cavalry with anti-tank rifles. If you can block production on planets (based on Stellaris's mechanics), if you can relatively easily destroy ships with enemy landing troops (based on the same mechanics), and so on, then ground warfare logically becomes secondary, and space superiority becomes primary. That's all.
If we're talking about how it will be in real life, then everything might not be so obvious, including the non-obvious point that such reasoning about space wars even makes sense in a very distant real-world future. But in Stellaris, that's how it is.