FWIW, I don’t think that you necessarily want/need screen casting for this. All that you seem to need is an app to add audio notes to a (PDF or ???) file.
In my dim memory is a vague recollection of an app that would do this on an iPad for PDF files. Search “add audio notes to PDF iPad” for starting points.
If you have setapp then capto is included. It might do what you want.
I just used it to demonstrate a bug I had found in a piece of software. It will capture your entire screen or just a window, record using your microphone and highlight things like mouse clicks.
It can, though I’m not sure it does it by default. It’s been a while since I played with it, so my memory’s a bit fuzzy, but you might want to experiment with both the built in screen recorder and Capture (from Techsmith).