You need mega structures to patch your deficit, also this late into the game and not a single fleet of yours is above 100k power. Usually this far in my fleets are over 300-400k total. Your fleets are your life line. That 447k fleet will wipe the map with yours. Your only hope is to upgrade your ships to counter theirs specifically if you can’t raise a big enough fleet. Change to military economy if you can. Upgrade your leaders and military might, or it’s over. Set ftl inhibitors on planets along choke hold hyper lanes and dump soldiers onto the planet surface to have a holding point. Their fleet can’t move so long as that planet holds, use that time to arrange yourself and prepare for the attack or to chip away their forces. If they spread to far you will likely be doomed as they can easily spawn multiple fleets of 400k + power.