That's the million word question isn't it? lol.
Guardian rebalance would be a better use of your time, but its your time. I'll help you out with setting up a test on guardian viability.
I would start somewhere along the lines of thinking of reducing the logistics slots/per resource cost to about here and work from there:
Resource cost per logistics slot
Resource Guardian Guardian reduced cost Domina Hoshiko Cielo Overseer Subverter
Credits 75 60 100 87.5 70.8 64.2 80
Metal 15.7 8 20 7.5 4.16 15.7 16
Crystal 17.1 4 20 12.5 15 14.2 16
I expect it to be a little extreme of a cost reduction, but then thats' the fun of balancing and what devs get paid for. I think this would help the guardian's unit price to offset the large cost of the shield upgrades. If it fails to do this then a further price reduction is in order, maybe something around 55 credits/7 metal/4 crystal.
I lowered crystal requirements since advent are the least likely to develop a strong economy outside of an eco spot.
The point of success would be to do a spam test on vasari and advent close quarters and determine via experience whether or not the advent fleet gains enough benefit from increasing the durability of their capitals to give them increased time to escape. Also your fleet shouldn't have very many guardians at all.
Honestly a better way to balance would be to just do a hard reduction in cost for the advent shield technology as a racially unique feature. This would be a much more elegantly simple way to approach the problem, but then I'll reiterate that the devs don't approach editing the research trees that often, even though in this case it would be beneficial.
To test advent fleet attack capability, try an enemy homeworld and see how long a group of 30 javelis frigates take to wear down the capital ship (standard effective dps group in multiplayer) with guardians and without, using a progenitor, then a halcyon, then a double halcyon or a progen halcyon combo (4 common multiplayer start combos).
Then time how long the advent fleet can survive without manuevering and then you'll get a good idea of where to balance.
So your data chart should look something like this:
Seconds to die either from 14 kanrak assailants+ space egg lvl 2(colonize + armor reduction/and or warp prevention)
or 30 Javelis Lrm Frigates + akkan lvl 2 (colonize and ion bolt)
Times are in seconds of survivability, also keeping a tally of the cost of the ships in question would be helpful. Progenitor would know colonize + shield restore and halcyon would know extra fighter squadron and attack speed increase.
Capital ships No Guardians(control group) Guardian 1 Guardian 2 Guardian 3 Guardian 4 etc.
Halcyon
Halcyon x2
Progenitor
Progenitor + Halcyon
Bleh formatting isn't workin right.
There are quite a few more variables, the only thing we are really going for is to determine the effectiveness of guardians being able to support an early advent attack or defense using their capital ships against known effective quantities.
Also, showing the total cost of going guardian and spamming a few of them when compared to other fleets.
Guardians don't have to be super effective at helping the fleet early on, but they do need to be good enough that they can immediately begin to help capitals survive the early game snipe. Vasari sniping will still be effective, but perhaps it might take a few more kanraks, and I believe that a vasari fleet when facing a progen + guardians + repair platforms at the advent Homeworld should be able to hold off 30 kanraks from destroying a capital ship for a long time. Considering vasari advantage being able to build a starbase in a grav well as a decoy to DPS the vasari still have a large advantage and are therefore just getting a small reduction in capability, meaning the advent capitals can stand up to them better so they don't have to constantly retreat for self preservation.
Well, as someone who plays the Advent in MP (I just do SP), what would you recommend (percentwise)
try 300% on all cap ships and go from there, would be interesting and my counter argument to buffing cap dps becomes less effective the larger the DPS boost(yes i'm trying to make your test successful at the cost of my own position). It very well could mean that a significantly large damage boost would give the advent capitals the something extra that they so need so as to justify their protection early on by buying guardians the normal way at the normal price.