Which IVs for Y!Tiki are better?

With a -Spd, you might as well make her an enemy phase unit. If you do go with the +Def and give her Steady Breath or Steady Stance for A and for B go with QR. And give her Bonfire.

both boons are really good tbh

If running her with DC or against fellow dragons then +atk is better as it will allow for more OHKOs.

If running her as phy melee unit then +def is better

I'm just calculating here using the IVs because I don't have Y.Tiki, but with -spd you're not going to do much attacking except OHKOs. If you're going to use this new anti-dragon breath, I'd consider going for OHKO with +atk. 34 atk IV + 16 mt + 5 from weapon effect = 55, and you could add death blow and even a atk+3 seal for 6-9 more power. That'll be enough to bust even some non-dragon reds. Your enemy phase will suffer a bit if you go with death blow, though, so you might compromise and do fury or pick something else that works on both phases like atk/def bond. Breath of fog has no cooldown penalty, so with the bond the QR bonfire will be good for (32+5+5)/2 = 21; meanwhile, you can still initiate with 60atk.

If you're content to sit back and bait melee units, then +def is probably better because 31 atk IV + 16 mt + 5 from weapon effect = 52, which is definitely enough for an enemy phase unit in my books, especially one that targets res. But then I'd probably go with lightning breath, because DC is really good for baiting and she has the res to do it.

If you're going to go with lightning breath then I would still go +def because you're probably depending on a special on the second riposte for the KO. Bunker up, leave them with some health on the first riposte, eat their double, aether back and pray that you'll regain the QR threshold.

One thing I forgot to mention: if you plan to pull for her anytime in the future (and we're probably about to get a chance), you might change her IVs, so plan ahead before you give up 200 dew refining breath of fog or sac any 5* for some breath skill or aether or atk/def bond. Don't get too excited just because the weapon is new.