-Spd Tharja

Well...do you have a +atk Leo? If so give him the blade tome+ for cavalry emblem.

Unless you want Darting Blow 3 on someone, she has no other uses. Without some Hone Speed user or an Eirika/Ephraim support her performance will be lackluster due to the large number of double hits she will miss out on vs a speed boon.

That depends on you and your team. Anyway: if you like Tharja but not your boons/banes you should just wait for a better one and merge the first one into the new one. Keep using her during that time, she is a good unit so why not?

She will never be optimal but with new modes like Arena Assault and Chain Challenges it's worth putting some investment into units with suboptimal IVs.

So, if you want to continue to use her instead of using her for SI.... First, -SPD is probably the worst bane she could have. The insane power of her blade tome would mitigate -ATK when buffs are up but -SPD cuts back on the units she can double / increases the units who can double her. Still, it pushes her SPD down to 31 base...which isn't great but isn't horrible either - that ties her with neutral Mae and Sonya. I'd recommend getting it back up to neutral with Fury and selecting a breaker skill for her B slot to ensure some doubles (popular picks would be Green Tomebreaker or Sword Breaker - don't see the point of Red Tomebreaker when they are so rare). She also ends up with 26 DEF...which is decent, especially after a Fury boost, but isn't going to tank many direct hits.