I haven’t used an older iPad on any version of iPadOS, but
@rui no onna has mentioned that starting with iPadOS 13, the 9.7-inch iPad Pro’s standby time has been horribly affected. I’m inclined to blame that.
Do you have any numbers of screen-on time when you’re actually using it?
In my extensive experience with iOS devices, and despite the vast majority making claims to the contrary, battery health is completely irrelevant. The best way to predict battery life is by taking into account usage, brightness, and the iOS version installed.