This issue is still worth addressing.
MacOS has a touchbar button that opens a menu of all the input sources. Is there a way to access that in BTT? And to customise it so it doesn't take up so much space?
I use four keyboard layouts plus handwriting on the touchpad. The default MacOS way of switching between them (press the fn key) is both slow and confusing because it's too easy to overshoot, and the tiny little scroller display that pops up is too small.
Update: I added a feature request at keyboard layout switcher