AST's weakness in end game

[NamoNanamo]

Using CO on just a party buff, even if it is AoE Balance, is a huge waste except for a few very niche cases

It's never a waste. NEVER. It could become a problem if you are running low and using every Ewer with Royal road
Of course, it could be better on another things but it will not be a waste.

I just had an AST on Thordan ex who on one attempt was wondering why he didn't have any mana,

And ?
I'm playing Astro and I can do all the fight, without Brd. And using Co on a party buff. And still have no mana problem on the end of the fight.

THe problem isn't using co on a party buff or not, it's how you manage your mana.
I'm not wasting my mana and I know how to optimize co. You need to adapt