Glacies/Iceberg and Bonfire/Ignis

For me 30 in either case is the absolute minimum and even then I would probably still prefer Luna or Moonbow depending on the character. 32 and up is where you start to see some significant gains on these specials.

It really depends, dude. I don't think they're worth it if you have under 30 in each of those. On the other hand, Dragonic Aura nets you a consistent 15 Damage if you have 50 Atk (not that hard to reach).

Other observations:

Vengeance & Reprisal

Can actually be pretty good if you can benefit them with Quad > Desperation users and the like (maybe also Firesweep users). If you have just over 40 Atk (think, 41) you're looking at 20 damage off of Vengeance minus the 4-turn cooldown in skills like Dragon Fang. With Reprisal, you're generally looking at something of a consistent 12 damage which would roughly equal a Moonbow against a 40 Def (+Def) Hector. Unlike Moonbow though, the damage always stays consistent.

I use bonfire with my sheena, it works very well.

https://kagerochart.com/damage-calc

This should help answer your question! I think the cutoff depends partially on the unit (and even then, theres not a huge difference in kill count between the skills) as well as the team you're running.

Also, if you have buff/debuff teammates, take that into consideration as well. Luna/Moonbow will be less potent on an enemy with debuffed defenses, while Dragon Fang/Draconic Aura will do more when on a unit with buffed ATK, etc.

My general rule of thumb is:
> High ATK, mediocre defenses -> Draconic Aura
> Decent ATK, good/high defenses, decent/high SPD -> Bonfire/Iceberg
> Decent ATK, good/high defenses, low SPD or brave weapon -> Ignis/Glacies
> Low ATK, Low defenses -> Luna (brave weapon)/Moonbow (other)