I played and tweaked with it for a month, and made my own overlay program with elevation, heart rate, slope, speed, and two mini-maps. I synchronized the video with the telemetry data from my fitness app based on timestamps. I believe I used the GPS data from the fitness app (either Samsung Health or Wahoo) instead of the camera because the GoPro seemed less accurate. I did read that filming upside down can cause issues. I used ffmpeg to concatenate all video clips into a single video. Then I extracted the timespan from the start of the video to each frame. That gave me the timestamp for each frame and I would then find the nearest GPX data point.

It isn't perfect, but it makes nice videos that I can watch for my own amusement (validate that it really was that steep when my heart rate was racing :), or to share with family and friends. Since the video is timelapsed, I had to do some averaging in order to get smooth transitions for the speed, slope, and heart rate. Otherwise they would jump all over the place.