here is a suggestion for a couple new techs and a new ability for nearly worthless flak frigate.
allow the flak to target projectiles from the super weapons like the novalift. sufficient flaks (10, 20, ?) could stop the weapon entirely protecting the planet. obviously, there would need to be a tech to get the upgrade. and a tech another to detect the incoming weapon. detection at maybe 5 seconds out or something with upgrades to 10 or 15 seconds.
it would be difficult to use since you dont have the fleet capacity to keep dozens of flaks at all planets to protect them, but if used to protect a home world, or choke point it could work. plus it would bring purpose back to the flaks.
flak gun ranges dont change, they can just see and fire on the weapon before it hits the planet. this would require the flak group to be in the right position to stop the missile.