Made this after seeing a stylus plugin for KOReader which my old kindle paperwhite 4 does not have. But it does have touch so I thought it must be possible to do similar with fingers. Gave Claude reference of 3 such plugins and asked it to make one for me. Latency of drawing is way lower than I was expecting, basically I am not seeing any noticeable delay at all (see the video in repo).
Its vibe coded so I can't tell anything technical about it, but it is working for me and many people on /r/koreader. I can even scribble on a PDF and save those back to PDF itself.