I've tried a few techniques that seem to have worked well and thought I would share.
First, clean the glass. Both sides. Speckles of dust become beacons!
Second, no light on the camera side of the glass. I installed blackout curtains.
Third, camera settings. I use F2.8, 1/100 shutter, ISO 320, on a Sony RX10 IV. This means the camera is focused on a very shallow depth.
Fourth, the post production. I use kdenlive and audacity. I first run the audio through a noise-reduction filter. I then crop the edges. I then use a Bezier-curve to slam the black level down (so everything close to black becomes absolute black). I then flip (mirror) so you can (almost!) read my scribbles. I then use a chroma-key overlay and pick up the whitish-green marker and replace it with a neon green (see below video).
To add a bit of pizzaz, I sometimes overlay in objects or colours as well (you'll see that here for example).
Getting the lights and microphone setup was a lot of trial and error, I think i have it relatively dialed-in now.
For those who wish to follow the journey, please feel to subscribe to the YouTube channel, and of course, the company blog where I will promote them and add some descriptive text, some what like here.
Let me know if you have suggestions for content or how to improve the videos!