Love the tab islands and I use them all the time! But there is one feature that is missing imo ... not just in Opera but in any of the browsers / extensions out there... unless there is and I would love to know!
I tend to group tabs mostly by site/domain, like all my github tabs, all my AWS tabs, Youtube tabs, etc. and my idea is to have a configuration option (per user profile) to create a list of islands that are based on site/URL patterns. For example:
Island Pattern
----------- -------
Github personal https://github.com/my-github/*
Github work https://github.com/my-organization/*
AWS https://*.console.aws.amazon.com/*
Youtube https://*.youtube.com/*
etc...
Then, whenever a URL matches any of the patterns, whether I navigate to it manually, from a bookmark, with ctrl-click, etc. it would automatically be added to the matching Island! And this would be a feature somewhere in settings that you can turn on/off...
Now that would be sweet! 
Opera One - version: 110.0.5130.23
Update stream: Stable
System: Windows 11 64-bit
Chromium version: 124.0.6367.62