First, I'll preface this: I suck. Especially at air. So just keep scrolling.
Anyhow, I agree with Saktoth's list of issues, the interactions are both too simple and they have a bad curve over the course of the game.
To fix the curve issue? Embrace that this is a two-tier game and move the special support facs into the second tier. Air, Gunships, Athena, Strider Hub, and Missile Silo. Mark them all as non-ploppable "Support Factories" with 1000 cost. No more air-starts. Removes a bunch of the opening-action RPS from the game.
Second, remove pure-air-to-air units. The default for air is flex-AA. This makes an air-to-air game more interesting since there are more viable unit options.
Divvy air units up into three roles:
1) "Tanks". Slow,
extremely low-DPS, low-alpha antiswarm weaponry and can tank a tonne of damage.
2) "Strikers". High-alpha anti-heavy weaponry, and enough health to deliver it.
3) "Interceptors". Super-low-weight, raw firepower and terrible health.
Interceptors kill Strikers (interceptors are low-weight, strikers carry anti-heavy weaponry), Tanks kill Interceptors (riot-y weaponry and their low DPS doesn't matter when your targets are made of paper), and Strikers kill Tanks (tanks don't have enough firepower to kill Strikers before they deliver the payload).
So we have an RPS triangle. Interceptors are primarily defensive units because they'll die up front, but they tend to be the fastest and cheapest so you can cover more of your base with them. Strikers and Tanks are both offensive, but can be used defensively for riot or anti-heavy work, respectively.
Now, the existence of Tanks means that we need surface AA to properly prioritize. Hacksaws can not waste shots on Brawlers. So simplify sjeah's AA micro widget - repurpose the useless "return fire" firestate mode to automatically attack available units that are
not in badtargetcategory. Rename "return fire" into "discriminate".
For AA, the Screamer needs to be something other than Chainsaw++, and Chainsaw needs to be something more than Hacksaw++. This will reduce the problem of "welp, he has a chainsaw, time to give up on air". Make Chainsaw stockpile-based but with a finite stockpile... so theoretically it can be depleted with judicious use of shields and flybys and whatnot.
Screamer, on the other hand, goes back to its BA purpose - pure long-ranged area denial. Make it terrible for actual base defense. It's artillery for exerting your reach over more space.
Cobra's blast radius is a solution looking for a problem. Repurpose it for raw, unbridled DPS over a short distance.
Then we've replicated the triangle on the ground - the Cobra brings raw DPS, it brings down Tanks. The Hacksaw still takes out Strikers. The Razor takes out Interceptors... which isn't really a hard task, which is why it also sports long range and excellent survivability.
Now, for the individual units:
In the Plane lab, the Shadow is now a flex-AA unit. The bomb is replaced with a very short-ranged missile. It's your mainline Striker. The Avenger is your Interceptor, obviously. The Vamp and Stiletto are converted into tanks - exaggerate the vamp's "slows down with attack" feature to effectively make its speed only useful for arriving at the battlefield. Its weapon is converted into a Tankish weapon - a low-DPS riot weapon. Flame-bullets or something. The Stiletto is converted into a sluggish flying low-power Outlaw that pulses *constantly*. Just give it a fake weapon and have it incessantly outlaw-pulse. No refueling anymore.
For the Phoenix, I'd just gut the weight on it and make the bomb purely fireball damage -
not ignite targets. That means it's primarily useful against slow/immobile things. But at the same time, increases its total damage potential. Basically, Phoenix is used to stop advancing forces and demolish bases, but suffers from serious attrition when used aggressively because of its low weight. Ignition is the real problem with the current phoenix - it lets it one-shot massive low-weight armies since they all burn to death.
On the gunship lab, the Brawler loses speed and DPS and gets a pinpoint-accurate weapon. The Blackdawn gets a precision missile instead of a swarm of them. Possibly a widget/gadget would be needed to improve Blackdawn AI such that it backs off between firings so it won't have to tank damage when it really shouldn't need to. The Rapier is your interceptor - gut its armor and buff its weapon. Dunno what to do with the Banshee... that thing's a mess.
You can now stop scrolling and read the posts made by those who know what they're talking about.