FrogReaver
The most respectful and polite poster ever
First of all, it’s not unusual for different actions to have the same difficulty. Second of all, there is never any uncertainty with fireball, unless someone tries to Counterspell it with a lower level spell slot than you’re casting it at, and in that case you do have to make a check to resolve that uncertainty. Otherwise, you say you’re casting it, it works without a check. That is precisely how it should function under G&A.
You said the important part right here: "There is never any uncertainty with fireball". Therefore it doesn't matter what your goal is or even what your approach is. It seems to me that if neither your goal nor approach matter then your not playing G&A.
It works exactly the same way any other action works. The DM describes the environment. The players describe what they want to do. The DM decides what happens, often relying on the roll of a die to determine the results, then narrates the results of the character’s actions.
When casting fireball the player states what they do. They don't state a goal or an approach and then have the DM tell them if they do it or not. That's a big difference.
Now, that process of the DM deciding what happens, often relying on the roll of a die to determine the results? What exactly that looks like depends on the actions the players take. Casting a spell happens to be an action that has a lot of very specific rules about how to resolve it. Most other actions are much more reliant on the DM’s judgment. But both fit just fine into the same action resolution framework.
Casting a spell is a mechanical action. It's not a goal and approach defined action. There's nothing about the goal or approach that ever matters for fireball.