I've only gotten use of fighters when I played with a max+ tech modded db where I went beyond what vanilla allowed. The only way for them to survive is miniaturized shielding as shield hits seem to trigger shock damage

This is one of two major reasons I have suggested that shields should be changed so that their strength scales linearly but their recharge rate scales superlinearly, the opposite of what we have now. This would make small shields usable (and help balance shields vs. armor a bit better).

I did use them in one major planatery invasion in one campaign and after a lot of micromanangement to set them up as the radiocative beverage said they all got shot down and before they died scored almost no hits and most of those did nothing.  They are at most effective as the same tonnage of ground forces , require many, many times more micromangment and have the survivability of a snowflake in hell. Fitting ground support weapons into the missile bays of your actual space combat fighters is worse, they are less effective , at least as vulnerable and vital for your space combat force so you really cannot afford to lose them.
I restarted the campaign with a new strategy

You'd still have to consider the unfortunate mathematics of orbit-to-surface weaponry. A comparison I've done a while ago told me that my destroyer escorts fire 196 10cm railgun (10/20 damage) shots at 5200 BP for ~26BP/shot, and railgun fighters cost me 51 BP/shot, while it costs PW infantry 0.15 BP for a 15/15 shot, and 0.24 BP for CAP infantry to fire 6 such shots. Of course, a specialized anti-GU design (a beam base?) would be far more economical to field, but I don't think even that could overcome a hundred-fold difference in cost-efficiency.

Your cost math sounds like it comes from before the fighter pods were added. A large fighter (400t) with my current tech can have 6 size 25 fighter pods and each pod (if Autocannon) does 3 shots with roughly 4 times the penetration of PWI and twice the damage. To get a comparable effect from a ground unit, I would have to go up to a MAC on a light vehicle (quite a bit less penetration, but as close as I see) which is almost exactly half the cost of the pod.

A design with armor and theoretically good survivability using micromanagement comes in at 175BP plus 30BP for 6 size 25 pods so 205BP versus 15BP (62.5) for ground vehicles which do similar damage. So yes the fighter is 14 times the cost, but you get arguably simpler logistics and much better survivability (in theory). If you used a Ultra-Heavy Vehicle with Ultra-Heavy Armor and all HACs, you would need 3 to roughly match the firepower and that would be 360BP. So Fighters and FFDs are only as stupid (in terms of cost) as the big ground unit types, well within reasonable costs for the role play value.

With that said, I have no actual experience in ground combat with the new system, or using FFD at all. So my guess that the armor will give me the chance to pull them back to the carrier for repairs may be totally off, thus making the "survivability" completely irrelevant.

Well, you learn something new every day. Never though to nest commands, but I am definitely going to give that a shot!

To be fair saturation bombardment with missiles or beams is quite good for wrecking a planet, and beam bombardment is a good way to neutralise STO. however an FFD is not needed for that and the equivlant cost of infantry with PWI will do more damage than an FFD with a 50,000 ton battleship on call, and you won't waste hours of your life setting up the bombardment. The nightmare of ineefectiual micromanagement which is fighters linked to FFD I will leave out in case it gives me nightmares tonight

Aye, I'm a big fan of "terraforming" enemy planets with saturated beam fire to invade on more favorable terms myself, but engaging non-STO GU's with ship weapons is meaningless whether done with FFD or without.

But what about using "fighters" that mass over 300 tons and sport huge weapons? An airship that might fit right in on Highfleet or something. I'm waiting on the new version before trying this, but it seems like it would cut down on the micro.

You'd still have to consider the unfortunate mathematics of orbit-to-surface weaponry. A comparison I've done a while ago told me that my destroyer escorts fire 196 10cm railgun (10/20 damage) shots at 5200 BP for ~26BP/shot, and railgun fighters cost me 51 BP/shot, while it costs PW infantry 0.15 BP for a 15/15 shot, and 0.24 BP for CAP infantry to fire 6 such shots. Of course, a specialized anti-GU design (a beam base?) would be far more economical to field, but I don't think even that could overcome a hundred-fold difference in cost-efficiency.