Marth Theory Crafting

The skills are fine, not like if you could ever go wrong with these premium passives such as Steady Breath but I believe his B-passiv has to be Swordbreaker, no matter what. Otherwise he won't stand a chance against the majority of sword users used in Arena.

Also, for the C-passive, if you choose not to go for a supportive passive such a Drive Spd 2 or whatever, Threaten Spd is actually more useful than Threaten Def. Marth's atk isn't too high so a Def debuff won't enable to OHKO his foes anyways, while a Spd debuff while either let him follow-up attack or avoid it (and profit the rest of the team greatly at the same time).

Finally, I would say Marth is just underwhelming overall. His stat spread doesn't shine by any means, I'd rather build another 4-5 star rarity such as Ogma, Grey if one can afford 5 star rarity or Hana if you have a lot of feather to spend into low rarity characters.
His "niche" is his exclusive Falchion, which isn't even exclusive to him really and Lucina, Alm and Chrom all have a better niche at using it.

I have a +spd-hp Marth with Fury/Renewal originally built as an offensive medic but he’s actually got great overall attack/speed and bulk. I agree swordbreaker is probably required to fight other sword users, but - skills permitting - I’d build him more like Ayra than a defensive build like yours. Problem is, he doesn’t have access to the kind of premium skills Ayra turns up with (Ayras blade innate effect, regnal astra’s damage output, etc)