Eirika +Spd / -Def promotion?

Might not be a bad idea. +Spd is really good for her, and I think she still has some survivability even with a -Def. Not to mention, if you need a Hero who can provide multiple buffs at once, she is a perfect fit for the job.

If using the Fury/renewal build you will be near impossible to double and she can withstand one color or colorless one shots with ease tho brave weaponry will be her downfall.

The TA/swordbreaker build will also work if you need a hard red counter that can still deal with most greens unassisted as well but her overal stats won't be as high without fury, weakening her damage output most but you'll run moonbow anyways most likely.

She's more of a support unit so I'd only recommend upgrading her if you have a stronger carry unit to pair her up with (ex: Nino). Aside from that, she's a weaker offensive unit (especially with -Def) than most other sword units, so it might be more worthehile to invest into someone else if you need a strong attacker.

I upgraded mine with the exact same spread & don't regret it. I'm in the process of adding Fury 3 instead of TA tho, I think that's preferable to offset the -def... But with that spread she runs both well anyway: