Remember when this game was good?

[Nezerius]

How did Ardbert and his group caused the flood of light by felling the Ascians? Is that implying the concept of "good" somehow is an element and because Ardbert and company did so much good in the world that it broke its balance?

There's no implication of the concept of "good", just the Umbral and Astral elements. Actions made through an element pushes a shard's element towards it, with the excess flowing back to the Source. If a shard gets pushed towards one element too much, the inbalance triggers a Flood.
The Ascians were already pushing the First towards Light, in preparation for the next Calamity/Rejoining. But not enough to cause a Flood, to avoid another Void/Thirteenth situation. They would then use Black Rose as a catastrophic event to trigger the Rejoining, causing the Source to fully absorb the shard in an act of preservation.

However, they didn't account for Ardbert & gang being a bit too effective at thwarthing their plans, causing that last push to trigger a Flood.

I never said it didn't have any plot holes, just not as many as Endwalker does.

But yes, from my cursory knowledge the implication seems to be that by foiling the Ascian's plans they shifted the aetheric balance too far towards umbral (or was it astral since that same story flipped them around?). But don't ask me how or why our actions haven't caused the source to turn into the same hellhole yet.

What got flipped on the First was Light being associated to passivity, while Dark being associated to activity. Which would imply Umbral Light/Astral Dark, rather than the Source's Astral Light/Umbral Dark.