Hey Milo,
I really like your ideas and initial mock ups. It looks clean and not too cluttered.
What do you think about the bookmarks location? You reckon it would still work if you can place your bookmarks on the vertical sidebar on the left and have them as a tab group essentially so you can expand/collapse them? They would in essence act as pinned tabs similar to in Arc and as you've shown.
As for bookmarks in the horizontal tab bar, they would act as pinned tabs and open the webpage in place rather than opening in a new tab.
This was discussed in the below thread:
https://orionfeedback.org/d/5377-larger-iconshow-title-for-pinned-tabs-in-sidebar-favorites-in-sidebar
I feel like these two threads come hand in hand, hence why I am mentioning it.
What are your thoughts?