For the sake of argument, we're going to assume that the way this character uses Elven Accuracy (which, when a creature has Advantage, allows one of the Advantage dice to be swapped with a new die, at the user's whim—XGE 74), is that this character will always swap their lowest rolled die with the new one, which is the mathematically optimal way to use it.
Swift Quiver is always going to offer greater damage output
Unfortunately, while having Advantage, especially Advantage with Elven Accuracy, is a pretty decent boost to damage output, it's just not going to compare to having two additional attacks.
Below, I've constructed a table representing the statistically average damage output per round of this character, assuming no other buffs or expended resources than what you have specified, and assuming that with using Swift Quiver, one attack gains Advantage (and thus Elven Accuracy), and with Greater Invisibility, both attacks gain Advantage (and thus Elven Accuracy). I've also included single attacks with a longbow, using a normal roll and an EA-Advantage roll.
|Swift Quiver x4
|Greater Invisibility x2
|Longbow W/Elven Accuracy x1
We can see that having Advantage is producing a modest, ~9.6% improvement in overall damage for an individual attack, but it's just not going to compare to a direct, +100% improvement in damage when using Swift Quiver.
Note an important fact however: this improvement becomes far less pronounced at higher ACs on the target. Indeed, I extended the array to calculate for AC 29+, and found that at that point, the difference becomes less than 1%. However, there is no point where mathematically, Swift Quiver deals less damage, no matter how high the AC goes, and no published creature in 5th Edition D&D has an AC higher than 25, which is the AC of the Tarrasque.
So ultimately, the choice this character needs to make is whether the 50-20% improvement in damage is worth losing their Bonus Action used for other purposes, like healing allies or providing Inspiration. If so, they should use Swift Quiver. If not, they should use Greater Invisibility.
Full stats for this analysis were posted here, for anyone curious.