Hey, so I'm sure this is a long shot, but I am also desperately trying to figure out a retractable bridge just like this. After extensively searching the web this is the only thing I could find and its like from 5 years ago. First off, ingenious human who figured this out, you are a god among minecrafter and take heart that either I am bad a looking or no one else has figured this out in five years. Second off, I would absolutely love to understand this better with either a world file or video or even photos and steps, thank you so much for your brain, it is an asset to minecraft world!
As this still seems to be the only bridge design of its kind on the web and the world download is broken, I took the liberty of reverse-engineering Entity's design from the pictures and tweaking it to work in 1.12.