That idea sounds overpowered. There are better ways to help the Advent. Return the Progenitor colonization bonus back to 20-40-60%. Now that Illuminators have been fixed (effectively nerfed), lower the military lab requirement down to two. That should go a long way towards helping the Advent become more competitive with Vasari and TEC.
The change to the Prog probably should be done independently of other decisions, if nothing else to put it more on par with the Akkan's colonize...
I personally am not a fan of a credit decrease bonus, but it is easy to implement and the Vasari already have an equivalent, so we at least have a base line to work with (4% per level)...
In principle, Advent are supposed to have the worst economy, which in theory should be compensated by a superior military...no one is going to like this change, but if illuminators only had frontal firepower and that firepower was comparable to LRMs, no one would be complaining about Advent until late-game faceoffs against fully upgraded PMs...
Halcyons (bombers) and Defense vessels are still strong enough to repel invaders before tier 3.. I think they should give defense vessel some kind of ability like the Vasari sentinel's charged missiles for a more abusive advent at tier 2.
IIRC, the sentinel's ability only affects SC...the in-game text may say otherwise but the actual ability is different from the kanraks, and I think the sentinel's deals damage to SC each use or something like that...
One could make a "synergy" ability similar to that on the beam defense platforms, and give it to defense vessels...that might work though defense vessels already are pretty damn good...I really don't favor making defense vessels OP just to compensate for an UP ship (the illuminator)...I'd rather take the direct approach and fix the illuminator which is the source of Advent's weakness...
Alternatively, making LRFs comparable to LFs in their ability to kill caps/SBs would also solve the problem (and arguably another more general problem), but there would be some inadvertent consequences of that....changing the damage tables doesn't always work out the way you'd think...