Just hard link ~/Library/Safari/Bookmarks.plist to ~/Dropbox/Bookmarks.plist on your different Macs. Done. Except that, to my knowledge, Dropbox doesn't support file ...
In a support document, Apple says it’s ending support for Safari bookmark syncing on iOS 10 and earlier. Starting December 18, devices will require iOS 11 or later, iPadOS 11 or later, and macOS ...