+Atk -Spd Bridal Tharja Build

Opposite ivs as you and ive found that her atk kinda....lacks tbh. Just follow the recommended builds tbh. She will indeed underperform but it wont be a dealbreaker.

With her base kit she still reaches a nice 46 spd under optimal conditions.
Id give her qr seal so she can take care of other ranged units reliably on enemy phase.
And good ol desp on her a slot.
With team support she can run heavy blade. And fury is indeed an option for more reliable bulk.

I say keep her base tome. Otherwise regular +spd tharja (or even +atk) will be kinda the same.