Do you use split tabs at all? I've found hiding the side pane and having multiple split tabs open in one window is a lot great when I'm trying to focus on work. Also not something totally new, but I use their Boosts feature a lot more than I expected, it's great to quickly "zap" elements from websites that I frequent.
Getting used to Arc's keyboard shortcuts and command palette has made it pretty indispensable in my work flow.
Only tried split tabs briefly, the UX didn't feel great to me. I might not be the target market here though as I tend to have a lot of tabs and navigate between them with keyboard shortcuts very quickly. I also prefer to just use the OS windowing for things that split tabs would be useful for.
Getting used to Arc's keyboard shortcuts and command palette has made it pretty indispensable in my work flow.