Possible solution to this may be displaying the arrow above the favicon, the way it's done in Firefox's Sidebery:
And don't display it for expanded tree at all, since the indentation of child tabs already indicates the tree:
Or use VS Code-alike lines to highlight the tree structure – like in TreeStyleTabs with custom extension:
But I'm not sure, how well it would fit the Orion's design.
But any gains in space this way will be lost by the request to increase the title font from the other card
Which means, there will be no information loss if the font size gets increased 😀