We had a forest of openldap trees in 2002. Openldap had a replication integrated. Slapd (its more than 20 years ago forgive me if I get things wrong).
You could use a counter or timestamps to resolf conflicts.
How do you solve conflicts?
After being all in the Apple ecosystem for the next 5-10 years but wanting to use a Kindle I decided to use Kindle + Kindle App in iOS (iPhone and iPad), after reading I export the highlights and add them to a Notion DB of my read/reading list where I can easily review the highlights when needed.
At one point I think i will give up on Kindle as a book reading and go all in for the iPad and I will switch to Apple Books but I think i will still export the highlights to somewhere.