Best iv for Kaze?

I lean toward +SPD; whatever damage you’re giving up is balanced by more doubles and not getting doubled in return.
That said, there is such as thing as ‘speed overkill’, which I’ve hit a few times on my Kagero (SPD: 48.) Watching her leave a slow unit with 2 HP is infuriating, as all that extra speed did nothing but a single attack point would have.
My advice: if you can get Kaze to the low/mid-40’s with your build (by skills, buffs, or IVs), you’ll be happy.

I have Felicia+6 and I'm going to speak from this perspective. Quick overview on differences between the two units: Felicia has 7 less atk and 1 more spd, def, and res. Felicia also has the Plate, which generally gives her a guaranteed KO against mages whenever she doubles if she goes for +eff, an ability which Kaze does not have.

There's a few challenges the anti-mage statline faces:
- mages are actually going to outspeed you, or at least be too fast for you to double naturally without significant investment. If you don't use +spd, Nino starts 3 points ahead of you so you're going to have to work really hard to find a natural double, nevermind brazen Celica. So you might want QR if you want a one-round, in which case you still need to make sure you don't get doubled because your bulk won't always sustain two hits. The good news is that the super-fast mages don't usually have enough res to block his ploys, especially if you merge him, so you can usually block their doubles if you hit them with spd ploy.
- not enough res to survive quickened moonbow from Reinhardt (if he gets both hits, of course. If you initiate into him, he won't OHKO you with a moonbow).
- not enough atk to get through high def targets, especially when you need lower CD specials anytime desperation is active. With fury, you have 45 atk at neutral, and you can add def ploy to bring it effectively to 50. 38 def and 45+ HP is not uncommon on unmerged armours, which already walls you unless you have like 42 res behind an iceberg, and they gain both def and HP with merges while you gain only atk. This might not be so relevant if you're using him to feed kills to the bonus unit because his main job will be to ploy and maybe add some extra debuffs with windsweep. Carrot+moonbow+flashing blade is probably your best try, actually, because against high-def targets the moonbow will be good for 10+ damage and you get another 10 from the carrot whereas barb shuriken + iceberg may fall short of 20 depending on buffs, merges, and ally support. The only workaround I can think of is starfish + flashing blade + special spiral + glacies, which is extremely expensive but will seriously obliterate 90% of the units outside abyssal mode and at +10, most units in abyssal mode that don't have 40-ish def. For a +10 merge you could definitely consider such an expensive build.

Kaze also has the issue of getting completely blocked by TA+raven unless he's using CA or barb shuriken + flashing blade + glacies. Felicia doesn't need flashing blade if she uses Plate+eff; in either case, the point is just to hit so hard that the glacies will deplete most of the mage's HP and those few tiny points of damage you manage to squeeze past the 40% disadvantage will secure the KO. Alternatively, just get a teammate to deal with the raven tome.

You might like to check out the double ploy brazen atk/spd build I submitted to his page. It depends on glimmer to OHKO squishy mages as a way of getting around the speedy mage issue. I think it works decently without merges with +atk, but if you merge then it might not be optimal.

If you plan to use him in abyssal maps, I think you better use +atk if you intend to kill stuff (ex: with that spiral flashing blade glacies I offered). When you get him to +10 I think +spd might not be necessary except against super-fast mages, but it could potentially be convenient just for not having to concern yourself about this stat in order to focus on others.