Speaking from my experience in single player, against multiple Hard/Unfair AI unlocked teams:
Fortifying is a viable strategy.
Tactical structures do just what they are supposed to and do it right. They fortify your gravity wells. You can win with a half to third-as-strong fleet against an attacker if you have a properly fortified planet( This means a mixture of hangers, repair bays, aux. structures(nanites, antimatter, or planetary shields) and turrets ). And it works well on very large maps as well. Infact , its much more useful on larger maps where your fleet is spread thin. Advice on using turrets though: don't ring them around the planet or the edge of the tactical grid, etc, place them close together near where you've placed other tactical structures. One really good advent setup is placing 4 hangers, 2 regen bays with overlapping fields, antimatter charger,PJI, and the remaining slots turrets all clustered together closer to enemy phase lane.
Why fortifying is not a waste of time: it allows you to turtle

You can get away with a smaller fleet, where the credits freed from not building extra ships goes into fortifying, and the credits not lost to upkeep allows you do always have research going.
Again, like my previous reply says, don't forget the 4X part while you are fortifying. As a fortifier, it's ok if you expand SLOWER but don't stop expanding altogether; quick scouting is needed to capture strategic locations first, and go from there.