I think I'm starting to figure out Stellaris Warfare and why I've been sucking at it:
normally I'm conservative with my resources and so have large stockpiles of a bunch of things just lying around. But I think what I'm supposed to do is spend them like crazy to build massive fleets and hope the stuff I have will just make more for me. I way underestimate how many ships I actually need to fight, and thus end up being weak in comparison to other empires despite how much territory I control, worse I need to anticipate and do all this ahead of time, like before war is ever declared.

like the fact that the AI cheats by having such big fleets out of nowhere is the one gripe I have with this game, because I have yet to win a war because of it.