I’m not sure if you are aware but there is a rather simple solution to import/export bookmarks:
- Go to
brave://bookmarks/ - At the top right, just below the magnifying glass to search brave settings, there are three dots, click the three dots
- It opens up a small menu giving you the option to export (and/or import) all your bookmarks
- I’ve also set myself a reminder using the Reminders App to do it once a week and/or month, just to be safe
Sorry if you are aware of this, you didn’t mention it and from what you were saying, you were making it a more complicated process in my opinion, though I understand you might prefer a automatic backup, doing it manually might be safer anyway, but keep reading 
. You can use Sync but it’s not smart to rely on it solely as a back up as seen here (as well as other helpful posts about sync within that thread): Helpful Info & FAQ for Brave Users - #35 by Saoiray
I also recently suggested, if you want to join me and vote for this as well: Make Backing Up Back Up & Restore Brave Settings Easier & More Thorough with IMPORT/EXPORT Feature
Hope that helped