I didn't know you could do that with BetterTouchTool, but I'm really glad I discovered this so my TouchBar can now be useful. There's a repo of examples here and the settings to achieve that are here and here
Here's my setup:
I also want to add a group with common apps and sites, but I'm happy with the setup I got so far. You can set it to change to something else for every app, but for me it's going to be more useful if it's always the same and I can build the muscle memory for every control.