--- I don't see the problem here, or rather, why the problem is one of scale rather than balance.
Considering that ground combat happens at the scale of multi-million ton armies (100,000s to millions of soldiers, depending how you fluff your units), basically world war scales, I don't think its unreasonable to think that fighters should be able to operate on the ~1,000s scale. Beyond that I think I will let others who have used GSFs more than me address the practical reasons in more detail.
That being said...
NOW, being able to DESIGN them as ships, but BUILD them as Ground Units, with stats reflective of their class... not too dissimilar to how STOs currently work... THAT I could (begrudgingly) get behind.
I think this is also fine and my original suggestion was basically this. The problem with GSFs really boils down to needing them to operate in line with ground combat mechanics, not naval combat mechanics (which they currently do) - if we can accomplish this while preserving the flavor of GSFs and allowing unique class designs to flourish that is only a good thing IMO.
--- Ya know, having Ground Support Fighters be their own
type of unit, like Fighters and Stations, might solve this problem neatly AND simply.
Consider the following:
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This design is classed as a Ground Support Fighter for production, combat and planetary interaction
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This design is classed as a Ground Support Fighter for auto-assignment purposes
--- Imagine this in the Class Design Window for a moment. GSFs are
already flagged as such for the purposes of combat and planetary interaction, so why not production and auto-assignment as well? Having a GSF specific bonus for Ground Officers to use with FFDs would help alleviate the officer issue at the ~1,000s scale if implemented alongside some changes to how FFDs work. Having as GSF specific bonus for Naval Officers, and perhaps a Command & Control component for GSFs specifically would further alleviate this. As for production... why not have them flagged as a separate class, but built at Fighter Factories anyway? A separate designation means that a special Hangar type can be used for them, perhaps one that uses Number of GSFs rather than the total Tonnage of GSFs to alleviate the issue of actually fielding them.
--- Likewise, GSFs having a separate designation from regular fighters means they don't need to drain fuel from the carrier. But that would make sensor equipped GSFs the ultimate scout, since they would need no fuel support. However, since they don't need sensors to do their job, nor does having sensors actually do anything anyway for Ground Combat we can safely prohibit them from having the special GSF designation if they equip any kind of sensor. Since they also don't suffer firing failures, nor contribute to naval combat, we may as well go whole hog and say that they also don't drain any MSP to restock themselves. They SHOULD however, still consume fuel as well as MSP, since Ground Combat is an 8-Hour affair, and time on station should be an important consideration.
--- What about combat though? GSFs are, again, already flagged differently from regular fighters. That's how they avoid provoking STO fire on Ground Missions after all... at least I assume as much to be the case. So why not give them more HtK and Armor? Say double the Armor and HtK? And while we're at it why not reduce or outright remove their vulnerability to shock damage as well? AA guns do no shock damage to Ground Forces, so this would if nothing else help make that aspect of Ground Combat more consistent rather than less. To let the player know, add a little line in the Class Design Window that let's them know that not only yes, this design qualifies as a Ground Support Fighter for production, combat and planetary interaction, but also it gets special boni to Armor, HtK, Shock Damage or whatever.
--- To my knowledge, these changes would accomplish several things at once. They would let the player know that the Class they have designed is considered a GSF. Currently, it takes some diving into the forum or Wiki to know, so this would be an improvement even if it was the only change made and merely cosmetic to boot. Next, by not requiring the player to spend Fuel an MSP to support their GSFs, the burden of fielding large quantities of them is greatly eased. However, by requiring the GSFs to spend fuel and MSP like normal ships do, and further by prohibiting them from benefiting from this if they mount sensors... or indeed
any non-GSF components, you effectively remove the ability to exploit this. Of course, it would be prudent to allow things like Bridges, Engineering Spaces, etc... Kill the exploits, but preserve the flavor ya' know? Furthermore, by allowing GSFs to have a special class of their own, the addition of a special Hangar that tracks GSFs and ONLY GSFs, and tracks them by number of craft, rather than total tonnage of craft, would go even further towards facilitating such massive scales. Finally, allowing the GSF boni to HtK, Armor, Shock Damage or perhaps other things; WHILE telling the player of this via the Class Design Window would allow AA to be nerfed enough to make GSFs more useful.
--- In conclusion, with regards to the HtK, Armor and other such boni; I think that best place to start would be either the reduction of, or an outright immunity to Shock Damage
specifically from AA would not only help GSFs, but also give more consistency to Ground Combat. I could see a newbie thinking "Hey~ AA does Shock Damage... Time to exploit this against enemy Ground Units!" and being a bit miffed when they find out it only works against GSFs. Other buffs should only be placed as needed, if needed. As a further note, GSFs should also be eligible to be Fighters too; with any boni only applying to pure GSFs. This would allow players to RP multi-role fighters without being mechanically pigeonholed into having them be one or the other. As well, GSF / Fighter hybrids should drain Fuel and MSP from their carrier. In a similar vein, even pure GSFs should remain eligible for the regular Hangars that we already have, counting total tonnage of craft according to the existing rules when doing so. This affords the player some tactical flexibility and doesn't needlessly or arbitrarily reduce functionality. A player can RP that GSFs need "special" equipment that only GSF Hangars have if they wish. The Class Design Window should classify GSF / Fighter hybrids as Fighters, and instead of the line showing GSF exemptions and boni (if any) it will instead inform the player that this Fighter can perform Ground Missions.
--- That's my full suggestion. I personally think it'd be a rather minimum effort solution overall, since GSFs already have some sort of code marking them as special and as you mentioned before they already function completely differently to Naval Ships. Adding one or a few components, while granting them some logistical exemptions I feel is the important change while resistance / immunity to Shock Damage from AA is important more for the sake of consistency than for the sake of buffing or nerfing. As always, feedback is welcome.