We don't know how the relationship between gravity and dark energy changes over time as gravity decreases (from the rest of the universe), because one cancels out the other to a degree we don't know.
It is not reasonable to assume that as the universe expands more strings of dark energy magically appear to keep the density constant.
Einstein originally proposed the idea of a cosmological constant because it was needed to maintain a static, non-expanding non-contracting universe, and he called it the biggest blunder of his career.
There's no way that dark energy can keep a constant density in an expanding universe. If the observations are correct that the universe is expanding faster, it's because as gravity decreases, the universe only needs a smaller push to accelerate.
If dark energy is not at constant density, which doesn't seem likely due to the way other energy behaves in the expansion, and if it changes from a pushing to a pulling force in the future, like when it gets sucked into a black hole, then the fate of the universe changes to an endless cycle.