Except that in a short round they suck, this is the same as just nerfing them by 4 provisions or something ludicrous like that.
If Larvae are a problem the Greatsword capping solution would be the only sensible approach.
The issue with this however is the fact that the only meta relevant MO deck is Viy, which does not even need Larvae and just uses it because it is generic and good.
If Viy gets the deserved nerf of only strengthening by 2 (and maybe going down to 7 base strength) MO might just vanish from relevance, so nerfing them even further will only make things worse in that regard.
Add the folowing cap: When boosted to 5/6 points or more transform into a drone without changing power.
And since the drone has no effect it just remains at 5 or 6 points, no more thrive and it loses it's armor.
And since the drone has no effect it just remains at 5 or 6 points, no more thrive and it loses it's armor.
Of course there have to be buffs for other cards to balance this out, you could see what happened to Nilfgaard when they nerfed IF, Braathens, Usurper and VVM and didn't buff anything in return. But Endregas are simply way too strong and super hard to deal with. Maybe just the copy it spawns could get Adrenaline, or they can get a cap on their Thrive.
Adrenaline (3): Boost by 2 instead and spawn 1 drone on the top of the opponent's deck on each thrive. 
I don't think it needs to be super complicated. Like I said in the beginning, I would be more or less satisfied if they just make Endrega Larva cost 6 Provisions. It would still be one of the best 6-Provision units in Gwent, but hopefully, it would no longer be automatically added to every single Monster deck, even the ones that lack synergy.
Even at 6P it would still be auto included. Not because there are no other good bronzes, but because it would still be a worthy 6P card.