In my opinion, it's simple to balance, but would require a ton of work. Get a real flight model. Retain current levels of HP and armor and increase missile and main gun damage. Reduce flak inaccuracy and reduce missile lock time, because ESF are going to be going a lot faster. Like, a lot. The A10 "Warthog" has a cruising speed of 560 kph. The F35 has a top speed of 2000 kph. Even the Super Tucano, a regular turboprop aircraft has a cruising speed of 500 kph. There you go. ESF that is viable in both big battles and small ones, because killing them is dependent on spotting them coming in for their CAS run, which they have to do at an oblique angle or they'll crash. They can no longer do the arbitrary flips and drops, because inertia and lift. Air superiority matters more, because it is actually hard to spot incoming aircraft and uncontested air superiority allows the more damaging CAS free reign. A2A combat suddenly isn't a mess of who can do the cutest flips and twirls in the air and is dependent on real maneuvering and air tactics. Real air combat maneuvers would be a lot more exciting than what's in game right now. Mossies and reavers turn into dedicated energy fighters, while the scythe becomes the turn fighter.
A helicopter model could work, but I imagine most people would rather fly fast with a few armaments, than have Hinds or Havocs and need to hug the ground to evade AA.
I guess it's makes SOE squeamish to consider balancing by making things more realistic, but the root of the ESF problems is that they completely ignore physics. The mossie and reaver can do bizarre maneuvers that would stall any legit jet aircraft and cause any helicopter to plummet from the sky and I don't think even the scythe has the agrav that would make such maneuvers possible. That completely borks A2A combat and lets you do absurd things like use the lib's howitzer as an aa gun. The same borked non-existent physics let ESF hover with their nose pointed directly down, allowing it to do what should be the domain of Liberators and heavy armor. If they ever bring us gunship galaxies, what is the point of certing them if you can provide just as much fire support in 5 ESF? 1250 certs is easier than the 6000 or more they'll want for a special role galaxy.
TLDR: ESF are kings of all trades, due to PHYSICS.
I guess I should just go play ARMA3.