Perhaps an option to force a new row when you click on the collection menu? This wouldn’t work in “Library” view, but this should be possible in “Collections” view since collections are only sorted alphabetically there.
Or maybe add a “collection group” property in the tags? More complex to implement, but would be the best and more robust way.
I suppose what you’re trying to achieve is an option to structure collections. There’s already two existing suggestions discussing options to create different sections for collections (e.g. to distinguish collections by actors, directors, awards, origin…) or nesting of collections. Please comment/vote in those threads in order to help us avoid distracting or cannibalizing votes. Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.