I think that the main problem here is that the Pandorans are just as strong at lower levels as they are at higher levels; they just evolve a little slower. I think this has been a deliberate decision by the developers, which I think should be reconsidered.
I see a lot of posts from experienced players complaining that the game is way too easy.
Speaking as a casual player (still with >250 hours of playtime thorugh the years) I find the game very challenging and quite frustrating at times, playing at Veteran difficulty.
Reading proposals to make Sirens and Scyllas immune to the only “weaknesses” they have seems just wrong to me since it will affect players on all difficulty levels. The only effect that it would make is that the majority of players will have their teams sistematically wiped off in early-mid game and abandon the game.
Please do keep in mind that not everybody plays the game multiple times and analyze every possible ability combination in order to find the most destructive. I see comments from players who played dozens of times. Of course you find the game easier, you have had a lot of experience playing it; but how about the first time you played? (Considering you did not go searching for tips/tutorials)
Most players will try the game at most one or two times. If they get frustrated because at lower levels they keep getting their team destroyed, they will surely won’t consider playing anymore, and possibly leave a negative review.
What I think should be done, is to ditch this concept (from the wiki):
However, the enemies on all difficulty levels are the same: they have the same stats, mutations and abilities, and the same AI
Also, RNG is not adjusted for difficulty: you have the same chance to hit and of being hit on Rookie and on Legend, and the same chances with Exploration Events
Pandorans at lower levels should be weaker, and possibly limit Sirens to one per mission (limit incread according to the level) and limit the possible mutations leaving the more powerful to the higher diffculty levels.
Only at this point you can consider giving more immunities and making Pandorans stronger. However, as long as the enemies are the same at all levels, this can (and should) not be done.
Also, I think that the “Second Wave” options proposed should really be implemented to give more flexibility to all players.