What works well for me is a taskbar icon with mouse scrolling (Waybar pulseaudio module on Sway). You see 50% and a speaker icon, hover and scroll up or down to instantly change.
As a universal law, optimizing the fullscreen experience is a suboptimal goal. Generally, the best solutions are the ones that operate at the appropriate level, and if we optimize the fullscreen experience, then most solutions built for the right level are inaccessible to us.