RE: General building Q, how to fill a low spot
Any bondo type filler is going to be HEAVY.
The light weight spackles are good but have a weakness. Over large areas like yours, they're going to present a very weak surface. If you figure to solve that by fiberglassing over them, you wind up with a better result. But it's often best to fill at least some parts of large areas with balsa and then fill around them with the light weight.
They don't give iron-on much support by themselves. Day to day handling will show up as wrinkles.
CA can actually turn their weak surfaces into decently strong surfaces. It's good to make sure the CA penetrates completely into the subsurface however. And the CA adds as little weight as the spackle did. win win