Give the free, open source OBS Studio a look. It records window capture and video device capture you've positioned as frames on a canvas. I haven't used it enough to provide meaningful guidance to settings, but in my limited use it performed well. There is documentation available. There are laptop performance issues, which required me to set graphics to "Better battery life" in the Energy Saver pane of System Preferences.
Some observations:
- you may want to use an external webcam as resizing and repositioning of the video capture frame necessitates sitting off-axis to any built-in iSight camera.
- to maximize Keynote content and avoid letterboxing, you may find it advantageous to use a vertically oriented custom slide size.
- Keynote needs to be in window mode, i.e. not Play mode, with the Navigator open and a slide activated in it; use the up and down arrows on the keyboard to move to the next slide. You may want to hide the Inspector.
