Promote Soleil?

Yes, it is that big of a drawback. Doing 20*2 damage is a lot better than just doing 23 damage.

So, you need to maximize on the speed. 32 speed after her bane is terrible in the current meta, and her speed issues will only compound upon themselves since she would need buff support. And, since Speed Tactics doesn't exist yet, she'll have to get buffed from Light Breath+, Peshkatz, or PA!Azura's dance.

TLDR; Wait for +Spd or at the very least not -Spd.

If I remember right. That's one of the worst, you want doubles with fire sweep, even at the expense of attack

That's pretty tough since a speed bane puts her down to 32 base speed. You'd probably have to give her Slaying Edge, or Wo Dao, and give it the +Spd refinement to offset the bane. That, mixed with a Spd +3 seal would put her at 38 speed, but even that's not really that high compared to what she could do.

You could get someone that has Hone Spd 3, that way she'll be at 42 speed, which is doable.

It's not optimal, but it's not impossible, either.

legit, build any soleil that isn't -atk or -spd. Mine is +res -def and runs fine. Its good to have +spd or atk but they aren't needed per say. you just absolutely can't run with -spd, its too inconsistent.

She is a unit that can be used with any IV honestly. Promote your current one, give her a Brave Sword+ and deathblow or even just atk+3 and you will be fine, she is available at 4 stars, so you will definitely get a better one to merge into eventually.