On Floorp, I used one of the old UIs from the settings, with bookmarks, panel and everything else disabled.

On Vivaldi, I use Custom CSS so I only get the tab bar (I’m experimenting with having it on the left hand side so I can see all my tabs and navigate more easily between them, with Ctrl-Up/Down to move focus, and Ctrl-Shift-Up/Down to move the tab in the list/stack)

Edit: Correction: On Floorp, I also got an address bar. On Vivaldi, I use a custom keybind to get the address bar to show up when I need it.