Gah, I am going to get sucked into do it, aren't I? Awell...
A class that has access to the entirety of the flavor of the month weaponskill is overpowered because it is always left with an avenue to exploit. If the exploitation is ineffectual then the use of that weaponskill itself is moot.
You say it sarcastically, but you're catching on at least. First Order Operators, is the term. The elemental system creates FAR more opportunities for them than a weapon-skill system, however, though it's just as suspect.
Even a very small exploit in a certain element is not moot if you built your character in that particular element. If the only aspect was damage then you might have a point. Make those spells have additional properties such as Bartel and you yourself suggested. Monotony is having single tiers of spells and hitting the same button over and over. If you have to select from multiple choices depending on the situation, there in lies depth. A decayable rate alleviates the need to spam as well.
The damage curve isn't needed in this circumstance, however, and all it does is fight against the very mechanics suggested in terms of priority. Again all your 'small exploit' at that point serves is to needlessly expand your spell list. For an elemental damage system to be a matter of relevance, it has to be significant. Otherwise it simply dosen't need to exist. There are better ways at that point to make the elements stand out, as been suggested.
Enemy design is the driving force behind what is overpowered or not, not the class design. If someone puts all their eggs in one basket for say Thunder. And every enemy is not weak to thunder or some absorb it. The thunder which was your strength in the last encounter becomes nulled in the current encounter. Just do not make the difference in power so grandiose between each element. That would lead to parties excluding or including you based off that. Create a balanced global enemy design where weaknesses differ and it balances the rest of the world.
You keep mentioning enemy design and class design as if they are relevant components to the discussion as a separation. We're talking system
design. The moment you really separate the two you lose ground on the argument.
When you design an enemy, you are effectively designing with or against the core system mechanics you designed.
Elemental wheels create a very regimented set of exploits within the core system itself. No matter how many times you design mechanics create gateways, the elemental system always boils it down to your base damage equations, and the fact that they have a capitalizing modifier. You would have to design mechanics specifically against
the very exploit you designed in the system to begin with, every time, without fail. To balance that out with encounter design.
That's a lot of unnecessary work for the illusion of combat depth, and your player base is going to see right through it and pick the first order operators anyways. In the case of a class with the full of the elemental wheel, they are now a shoe-in against any class that does not, especially when their core design, such as Black Mage, is built purely to deal damage, and lots of it - they become that first order operator at a much higher percentage than someone who does not have the same number of short cuts through the gated system.
The best idea in encounter design (Monster design as you call it) as far as creating a dynamic and open combat idea is essentially objective based encounters. Less about meeting out the best DPS, more about meeting the side requirements in the fight that distract from and/or threaten your success or level of it. At this point, default elemental weaknesses are essentially pointless, because you can create a checklist that composes multiple options. You can instead create more global vulnerabilities for meeting the objective efficiently.
SE has already begun to display methods of combat that is dynamic and thrilling without the need of specific elemental weaknesses. Given the construct of the game's lore and how the skills are distributed between jobs, there's no real sound way of implementing elemental weaknesses without it harming other parts of its mechanics, lore, or both.