@san4ess In the conversation I had with the moderator leocg, he and one other person told me that there is no internal backup for clients, unless you manually make backups yourself or use opera's own "sync" .
And so that you don't lose even a single bookmark, he recommended that you make backups of your bookmarks and restore points from week to week or month to month, or if you prefer, on a date that works for you.
To back up manually:
1 º Go the way you gave me and copy the bookmarks, if possible the bookmarks.bak
2 º From the opera itself: Menu>Markers>export markers
3rd with Bookmarks Import & Export Opera extension
But, I had other possible solutions in case you happen to lose the markers.
1 º Use restore points, on a date that has its markers
2nd If you don't want to restore the whole computer and get only the bookmarks,
use the ShadowExplorer tool, which basically takes the last restore point and shows you the folders on that date
3 º Third and last, if it doesn't work here, it's up to you and continue without the markers you missed. The solution would be for you to download some software that restores deleted or modified files
Observation: Until then I thought that opera's sync was for that, but the moderator Leocg himself told me that it wasn't, so yes, the recommendation is to make simultaneous backups manually.