I’m not an expert in that, But I also came to comment about something like zfs. I’m currently using btrfs and it works great for backup. The idea is that the filesystem itself contains the information what changed as far as I understand. And therefore the send operation are very lightway. As they are so lightway it would probbably be possible to also just do manuel sync(e.g. when you finished working for the day. However I can currently not think of abdirectional way where you have changes at both places and then merge them.

E-Ink and Ebooks in generell. Maybe not all the shitty Software/DRM that often comes with them but the technology itself is amazing.