I wanted to make one for Vivaldi but it doesn't use Chromium bookmarks API and there was a discussion about it already where it was said to be implemented with the option to sync on a self-hosted server. (https://forum.vivaldi.net/topic/5816/sync-settings-and-bookm...) I've also been working on one for Chrome but haven't had time lately to work on it.

Firefox has the ability to save bookmarks and all settings for that matter to a local folder upon closing the browser, you could then use an open source and hopefully encrypted sync tool to replicate those backups. Might be a bit clunky at first but if it catches on I'm sure one of the open source sync tools will come up with some sort of integration.