I know this feature exists in Vivaldi, but I love a lot of the current Opera features and don't want to change...

AND it shouldn't just be "tabs on the right side": it should be more like the feature rich (but outdated) extension "Sidewise":
https://chrome.google.com/webstore/detail/sidewise-tree-style-tabs/biiammgklaefagjclmnlialkmaemifgo
-> Tab-Tree, Hibernation, Folders, Titles, Highlighting... a "sidewise for sidebar", so to say

I haven't found anything like that for the "extension sidebar", which seems to me would be the best place, IF one could switch the sidebar to the right...

Thank you,
dlder

PS: If any reader knows of something like that, PLEASE let me know! Cheers!