@drakodan Open opera://flags/#sidebar-site-panel [copy - paste it, or simply open opera://flags and search for "sidebar-site-panel"] and set the highlighted flag to Enabled
Then you can add up to 5 custom sites to sidebar in sidebar's settings menu [triple dots at bottom]
Btw. it's maybe an experimental feature, since its disabled by default, but [for me] it works nicely.

@drahcirw You can de-attach twitch chat by clicking on cogwheel menu under the chat message input-box, and there's a Popout Chat button to do it. It opens the chat in new separate window.