My starter floated, but my levain floated, in slow motion disintegrated, and then sunk like a stone. I've been told this is a sign of a levain that is too mature. (I've had success with levain floats--gently taking it off the wooden spoon and using room temp water).
Is there a way to save the levain?
Smell is not strong. Aeration is moderate.
Float test is not even close to foolproof. Your levain could be just fine, but we'd need whats and hows to tell you more.
What are the markers I should be looking for? I'm trying to understand how to catch the different stages of fermentation at their peak.
My suggestion would be moving away from 100% starter, for a while at least. It's very convenient to mix, yes, but its peak is not always obvious. Liquid starter, in 125% range is easier to catch at peak because of its bubbly appearance and characteristic foaming. Stiff, 60-70% hydration starter is easier still, as it will rise significantly, dome, and may start sagging.
PS. I do not know who championed it, but until a few years ago float test was pretty much never mentioned.
The float test can certainly be very misleading and I never use it. If you have 100% hydration levain, it doesn't hold the gases as well as lower hydration levain, so it is more likely to fail the float test.
You'll know when your levain is ready to use when it just crests at its maximum rise and barely starts to fall. At that point it is starting to flatten at the top instead of being convex (rounded). A bit more mature you will see slight streaking at the sides of the container from the levain as it has fallen.
Benny