If I switch to Grid for layout, don’t I still need Bootstrap for other things, eg toolbars, buttons....? Or is there a more lightweight choice for those?
Unless you want to create your own styles for components like buttons from scratch, yes, you will still need a CSS framework. There are a plethora of such frameworks available today, many of which are lighter than Bootstrap (and don't require JQuery).
I'm very familiar with Bootstrap though, so I'll personally continue to use it for a long time to come, unless there is a really compelling reason to use another framework.
