While I'm in favour of asynchronous feedback like this, placing toasts nowhere near the button you've clicked is confusing when you get to bigger monitors. A toast in the bottom left on a large, widescreen 4k monitor can literally be half a meter away from the place you clicked, so the toast might as well not have been there. I myself have lost the progress notification for a file copy in KDE because it was placed all the way in the bottom right corner, and my screen isn't even _that_ big.
A little popover near the button makes more sense. Or, in this case, simply disabling the checkbox until the asynchronous action has completed, and using the non-disabled state to indicate success (or show a useful error message when the operation fails).
Right, so the issue is placement, not really toasts. I would prefer the pop-over you mention to toasts, but I definitely don't want to lose asynchrony.
A little popover near the button makes more sense. Or, in this case, simply disabling the checkbox until the asynchronous action has completed, and using the non-disabled state to indicate success (or show a useful error message when the operation fails).