F!Corrin build recommendations? Speed the way to go?

Alright, so it really depends on what you want to go with here. If you go +Spd with Spd refinement she's hitting 44 Spd at +10 before any buffs, which is pretty decent, so with either of the builds I recommend going for this. She's doubling stuff because she's a dragon that's actually fast. Tiki's Spd is unredeemable so she might as well just go -Spd because she benefits more from that than any other bane.

Corrin also has awful Atk so she really needs to be able to double consistently

Distant Counter Dark Breath is what I'm building on my +10 Corrin. I don't have Distant Counter yet but even without it, it's a nice build to use for supporting allies or just beating stuff in enemy phase
Dark Breath is quite nice because you're making them go -7 Atk/Spd in player phase and going for the KO in enemy phase, and you're able to counter regardless of weapon type unless it has Watersweep. This is better in levels where mages are more common, and is really good for out of arena content

Another way to run this kind of set would be Steady Breath Aether with Windsweep, where you're trading the ability to counter mages to make an almost unbeatable physical tank and you're getting your debuffs for free, but you lose your Distant Counter.

Personally after using her I think she'll prefer Distant Counter of some form, but both sets are quite good. Regardless of which one is used though, debuffs work really well with dragon class buffs like Fortify to really minimise what damage they actually do to you

Lightning Breath is more if you're wanting both Steady/Warding Breath while keeping the Distant Counter, so you're really just going for the standard dragon with it being fast instead. The notable thing here is that you lose your debuffs.

As above, +Spd is good on her. Dragons tend to be enemy phase so having a fast player phase dragon can help against Bold Fighter and dazzling healers. I've built her for the Dark Breath debuffing setup so either she takes the weakened enemies on enemy phase or is swapped/repositioned for an enemy phase built ally. I currently have her +Atk but I'm looking for +Spd on her as well as female Kana (little hope there) to join my +Spd Y!Tiki as player phase dragons with Nowi, A!Tiki and Myrrh/Fae as enemy phase.

Her speed sets her apart from other dragons that allows her to be able to do a mixed phase playstyle. Her player phase debuff style allows her to be like a dagger unit but has the bulk to back it up. With the right skills, she can effectively debuff all four stats. As a matter of fact I have set aside 11 Corrins for a merge project sometime.