So, number crunching.
I decided to take half the points out of Avenger and allocate them into Ether Mastery. With the N's evasive abilities, I can afford to have them switch places, and a Z is probably the last character who will benefit from 200 heals anyway, even if they weren't the most likely to die in the first place. And Ether Mastery is objectively better than the elemental masteries (2.5% increase in average compared to 1.1%), even for a single-elemental Z, let alone a triple elemental one.
But then I started wondering if I shouldn't take 9 of these points and put then into the star skills directly (only 9 of them because symmetryyyyyyy). So I started calculating.
With the initial setup, 1Star 10EM, the Z would do 50% damage (40% base damage * 1.25 EM bonus).
With 2/7, the base damage would increase, and even with the lowered bonus, the total damage would still be slightly higher (44%+1.16=51.04%)
With 3/4 we get 48%*1.08=51.84%
And 4/1 gets us 52%(Already better than the initial setup)*1.02=53.04%
So that's a net increase in damage of 6.08% there. However, it also increases TP consumption by 3TP to 11TP per cast, a 37.5% increase. It's not likely I'm going to use the stars often, but when I do, I will probably not even bother to use Dark Ether either, so the cost is not negligible either.
In short: worth it?
E: I could actually take two more points out of Avenger and put them, along the last point of EM, into the star skills raising the damage to 56% at 12TP.