I feel like that would just make the Yda deception even weirder honestly.

I don't think Zenos necessarily needed to kill a main character. But the way the attack on the Reach was handled could have been done different to have a bigger impact.

For one, even the minor character that died there was killed not by Zenos but by Fordola. If I'm not misremembering the attack on the Reach was even implied to be Fordola's idea in the imperial side cutscene before it. Honestsly I kind of wish Fordola had been in charge and not Zenos, she had an actual stake in the outcome of the war story where Zenos was bored by it. Having the enemy been an ala mhigan who sides with the empire and a side antagonist who is obsessed with the WOL as a personal rival IMO works better than having the side antagonist be the one actually invested in the greater story and the enemy commander not caring about the war but only us.

One thing they could have done is had us forced to abandon the Reach. Then have us fight to reclaim it when we return to Doma. If we'd had to fall back to Castrum oriens or w/e it would have helped sell it as a devastating blow. But it felt really jarring to me that their SECRET GLAMOUR HIDDEN BASE was attacked but they didn't feel the need to fall back or relocate.

Zenos' fight itself should have been handled differently. If they MUST put in an 'unwinnable boss fight' they should have done it differently. One example I like to point to is the fight against the tiger crystal L'cie at the end of chapter 3 of Final Fantasy Type 0. You're desperately dodging and voiding his attacks, that will quickly destroy you, frantically trying to just not die.

In Zenos' first fight, it's not a difficult or frantic struggle to survive against a superior foe. It's a fairly standard instanced fight but with his health pool inflated. Which makes the fight feel like a slow tedious encounter even though it isn't really all THAT long. It doesn't feel like you're against a foe you can't help to contend with, it feels like something you could chip away at if the game actually let you and then suddenly the 'you lose here' trigger occurs and you lose in a cutscene.

Instead, I think they should have had Zenos going all out, with his damage ramping up and up, and having it end when you die instead of 'you got the boss to X% you lose now.'