@leocg Ok. I may be doing something wrong, or my Opera installation may have gone wrong in some way... The option does not change the way the bookmarks 'system'/page/menu is opened for me. The panel opens regardless, with an option/link at the bottom of the panel to open the full bookmarks page instead.

Clicking on a bookmark opens this bookmark in a new tab in both cases too, so the "open full page" doesn't change anything there, either.

Alternative solution in my case: I pinned the full bookmarks page as a tab, which shows up with a neat heart symbol with the rest of the tabs.

When clicking on sidebar setup -> opera tools menu, there is a "open full page" toggle option for bookmarks and history.
Setting these to "open full page" vs. "open in panel" makes no difference.
Both options open the panels in the same way.
E.g. (with "open full page" set) the bookmarks panel opens, and then one has to click "Open full Bookmarks view" to get to the full bookmarks page.

I tried reading an earlier post:
https://forums.opera.com/topic/54703/solved-default-full-bookmarks-view

I don't get it, though. Is this the right way to get the browser to open the full bookmarks page when clicking on the bookmarks/heart symbol in the sidebar? Or am I supposed to do something else?

Speed dial, extensions, etc. open just fine as full pages, though...