What version of Opera are you using? In versions where bookmarks had only recently been added that was an issue but they've improved it since then IIRC.
Correction:
Opera 28 does sync with Opera Mini on iOS. In the speed dial section of the bookmarks you need to copy to bookmarks from one device (e.g. shown as your desktop folder in bookmarks) to the mobile (shown as mobile phone Opera folder). It didn't work the first time but it works now, it least for one time sync, I didn't try how it works for adding or removing items but it's not quite reliable yet on Opera 28 so I don't recommend using it.