Any use for Ishtar if I have a well invested blue mage?

Do whatever you want. Ishtar is arguably better offensively because she doesn't have to rely on the brash assault gimmick in order to do follow-up attacks (and is just straigh-up better than Micaiah overall imho), but feel free to ditch her.

i'd say yeah. Isthar is probably the best infantry blue mages right now. and she's didn't even need a lot of investment, since her default skill are all perfect for her.

Use her, a reduced cooldown tome with +4/10 Atk/Spd with her base kit is incredible. I used Linde as my main blue mage but then she came along and took the top spot. Micaiah can do defense better so you could still keep her in use but Ishtar will overshadow her as offense.

Ishtar is a real goddess of thunder, best offence.

You could change Micaiah to a more defencive role, she has amazing res. Something That Ishtar lacks.

I agree with others, use her first and see how you like her and if you still feel the same way you can fodder her.

It's never a bad idea to have multiple units that can do specific roles. You'll never know when they come in handy for the likes of AA and CC.