Regarding the encoding efficiency, I imagine the problem is that the compromise in quality shows in the space dimension (aka fewer or blurry pixels) rather than in time. Users need to read text clearly, so the compromise in the time dimension (fewer frames) sounds just fine.

Nothing stopping you from encoding h264 at a low frame rate like 5 or 10 fps. In webRTC, you can actually specify how you want to handle low bitrate situations with degredationPreference. If set to maintain-resolution, it will prefer sacrificing frame rate.