It seems you have to be very aggressive and keep expanding in this game.
I'm pretty new and after losing the first few games, I found I need to keep pushing in early game, grab as many planets as possible and keep attacking.
You probably wasted too much resource doing this - 8 fully fortified worlds.
Probably better off to shore up the front line/ choke point and build as many ships as you can.
I build 3-4 repair bays, and 4-5 def platforms, that seems to be able to repel early game attacks, adding hangar when I can. I can hold off 1 on 1 (cap ship) fight and sometime 2 on 1.
You need to keep attacking to cripple their economy/ship building ability (destroy military lab.)
Just keep attacking and then retreat to repair your cap ship.
I only played mostly small and medium maps so far and this seems to work pretty well. As soon as I get the 2nd or 3rd cap ship, I can open a 2 front war and bleed the AI to death.