I just switched to opera gx, including on ios, and I'm appalled it doesn't have a bookmarks system. This is a fundamental feature that any decent browser should have. This really should have been implemented ages ago, but I'm hoping it's at least done sometime in the near future.
That being said, a temporary workaround I found from other posts is using https://www.sync.opera.com/ to access bookmarks on mobile. You can add this website to your home page on mobile, and if you have sync enabled with an opera account you can fairly easily access any of your bookmarks on mobile via the website.