Brave Sync i great which allows users to sync bookmarks and other data between devices but the con is if you lose all of your devices then you would lose all of your bookmarks.
Chrome and Firefox allow its users to backup every bookmark to their cloud service. This is great because if you have one or two devices and they break or get lost your bookmarks are recoverable. Also if your computer is off for days and in the Brave browser on your mobile phone you have added 30 bookmarks and then your phone breaks you would have lost those 30 bookmarks since your phone was not able to sync with your computer being turned off.
Here are some suggestions to solve this issue.
Brave does what Chrome and Firefox does, Brave users can create a Brave account and everytime a bookmark is made it will be backup to their account. This can be costly since millions will be using this feature so I suggest only backup up bookmarks and not history, cookies, or passwords. Also to reduce data size I would suggest have a client-size compression/decompression which will make the data stored on the server smaller. If this is the route to go I highly suggest having the bookmarks saved in the cloud encrypted using client side encryption making it impossible for Brave to read bookmark data.
The same as option one but allow anyone to create a Brave cloud service that can backup bookmark data. This way users can choose to use service A, service B, etc. It is the same as suggestion #1 but Brave does not manage the bookmarks instead third parties will. Some features will be needed in Brave for this to work by selecting the server, account ID, password so users can connect to the service and login. Again I highly suggest having the bookmarks saved in the cloud encrypted using client side encryption making it impossible for these third party services to read bookmark data.
Have a setting in Brave for all versions (Desktop & Mobile) for when the user adds, edits, or deletes a bookmark it will automatically export the bookmarks into a certain folder on the device which us chosen by the user. Now all the user needs to do is setup their cloud storage provider to sync that folder to their cloud storage. This is a more private approach since the user can choose how to handle this data and which cloud storage provider to use. It is also the easiest for Brave to add since all that is needed is a way to detect when a users bookmarks are modified and it will export the bookmarks automatically. I highly suggest having a option to set a password on the exported bookmark file to encrypt the exported bookmark file. This way if users are using a cloud storage service like Google Drive or something with poor privacy the bookmark data is encrypted and requires a password to access the data.
These are my suggestions to ensure bookmarks are not ever lost and secure.