The inhabited systems even during the hayday of the star league will not even amount to 1% of the existing systems.
Regarding defense, I think stations have the advantage of cost. Sure, they don't move, and are more fragile, but any important system (yards, or political relevance) can be easily covered by 5 of them to defend a planet or yard at the cost of a half corvette.
I actually find it quite interesting that no one so far built ships to counter fighters - but then, no one but the FS has actually researched AC/20s yet.
Depends on what you consider a counter to fighters? The BEST Counter to a fighter is another fighter. The FS has their CVs, and several people have designs with alot of fighters on them.
Countering fighters with warship armament is challenging - if you want enough PPCs/ACs/Lasers to deplete a 100 fighter swarm with meaningful speed... thats a LOT of PPCs/Lasers/etc., and remember youll need to buy 4 for every 1 that's going to be in arc. AAA armament will probably climb. I reckon, based on my memories of Aerotech, that IN GENERAL it takes about half a fighters total armor in damage to kill it/send it running home, on average. So call it 80 damage to mission kill a generic 50 ton fighter.
Now, what I think is turning people off to fighters right now is the idea that once you layer on 200 armor on every facing your ship is 'immune' to fighters, and we are watching armor numbers climb right now. Still, I would consider:
1.) Older ships, smaller ships, etc. are still in service, and will be for a long time. Few ships right now have 200 armor on every facing. And AC/20s are not hard to get.
2.) Even with 200 armor on every facing, fighters are at least a threat to anything outside the armor belt. Radar and optical and IR and all those other fun sensors probably dont work through warship armor, and once the sensors are gone, a warship is a large, expensive, hunk of metal. Weapons and Drives may have SOME vulnerability, though much less. Now, Im not saying a dozen fighters will blind warships left and right - but you cannot just ignore them. Consider that Battleships have been sunk by fire that they should, formally, have been immune to.
3.) If the fighters aren't having to operate alone, but have warship support, they can capitalize on armor facings weakened or depleted by their allied warships fire. Its relatively easy to hide a damaged facing from the enemy wall - rolling ship is a thing, for reasons, and similarly you can hide nose or tail if your not as concerned about what direction your accelerating. Fighters swarming over a ship don't care, and will go after the weakest point. Few ships will reliably have 200 armor on every facing once the shooting starts.
4.) RE: Artillery Cannons - I don't see a problem with them, nor with adding them. That said, Im not sure what the use case is? They are -not- AOE on space hexes - space hexes are far too large. The best 'use case' I can find for them is after the invention of reflective armor (do we even have that on our charts? Either way, that turn wont come this year..), as they are sudden death to reflective armored craft - and reflective armor is itself super-effective against the energy weapons common in space?
