Tried Kagi a couple years ago. It was fast and I loved the zero telemetry. But Arc's organization... pinned tabs, folders, active tabs, spaces... was superb. And their mobile app is matching in excellence. Glad to see the sidebar being thought about here. It was my biggest turn off to using Kagi, the windows things or whatever it did with the total tab numbers instead of just giving me folders... And after spending two years with Arc, there's no way I would ever go horizontal again.
All to say, if the sidebar is done right I think a lot of Arc folks would land here for a faster and more private experience. Will return to this thread in a few months to see if there's been any progress.
Good luck folks!