Build for +Def,-Atk Ishtar?

Of course she's still viable. You could replace Swift Sparrow for Death Blow as she has enough speed with her tome, or use an Atk+3 Sacred Seal... But honestly, she can use Moonbow on her second attack in any round of combat thanks to her tome's cooldown reduction so the -Atk really doesn't hurt that much.

I have the exact same Ishtar and, honestly, she's fine. She's still extremely fast on player phase, 49 attack and 46 speed on initiation with her weapon and Swift Sparrow alone, not to mention her allies having access to C skills like Hone Atk, Drive Atk, Atk Tactic, etc.

She also has Odd Atk Wave...

So yeah, she's fine.

Give her Death blow and QP as her seal, or an ally with IP to reduce her moonbow to instant activation, now she can proc her special 2 times in her first fight.