Scramble turns are bad?
Its either that, or the first group will be killed off before they can act or cover. Or the first group has to be huge to compensate, meaning if you get a couple of misses you are fucked.
Here's a wild thought. And I'm not sure the world is ready for such a radical idea that has never been tried and totally wasn't a thing in just about every game from the 90s - don't have enemy groups. Have individual enemies with their own awareness and pathing who join the fight or don't based on their own perception.
And they should all have hand crafted names, appearances, backgrounds, unique voice lines acted out by different actors, and a tie-in novel in a PDF shipped with the game disk. But, and I know its a wild thought, that's too much fucking work. And though my example is farcical, yours is also obviously much more work than a studio is prepared to do today, given their profit margins, given how much other work they need to get done.
Designing encounters instead of "organic" big maps that feel real and the player can approach, and then the dynamic systems of sight and sound play on each other, and every enemy has their own alertness and will do their own action based on what they've individually seen or heard, and maybe if one enemy passes near another they can share info and... too much work. Realistically, it will just be enemy packs, and this has been the case for 20+ years.
Even when designing dungeons for something like Oblivion, you place encounters. The set piece fights are with packs, not organically moving individuals doing their own thing.