As others have already pointed out, this approach mostly protects the contents of your home directory from a cracked browser. It doesn't entirely isolate things

As others have already pointed out, this approach mostly protects the contents of your home directory from a cracked browser. It doesn't entirely isolate things at the X level. A more secure (and perhaps simpler approach) is to have each user run their own X server on a separate virtual console. The size of an X server isn't really significant these days. So each X server will be entirely isolated from the other using the regular X security stuff. You wouldn't need much of a desktop environment for the extra sessions. You might not even need a window manager, depending. You could have one extra session for your banking stuff and/or another for your sketchy streaming sites.