A good air-based counter to the ASF would be anything mounting an extended-reach air-to-air missile: https://www.thedrive.com/the-war-zone/40660/f-15ex-to-carry-new-oversized-air-to-air-missile
Presumably, a new plane would carry something like this in FAF. It could shoot from ~5-10x ASF's weapon range and the missile would have both tracking and significant AoE (maybe 10) to kill 10-20 ASF in one shot. Missile damage would be low (~4,000) and rate of fire would also be low (0.05, or 1 shot per 20 seconds) so, if this unit cost ~5x an ASF's cost, it could be weak against air XPs and outright countered by T4 bomber and CZAR. To make this unit even better, allow it to fire the extended-reach missile backways, thus allowing it to kite the ASF hordes.
The counter-play to something like this would be sending a few ASF in, or using these things to shoot down each other. The first is micro-intensive and small squads of ASF lose to ASF hordes. The second option is expensive and does little to kill bombers or T4 aircraft.