As there is no control nor links between Quotas and the total available space, it would be easy and very helpful just to add in the Users page a column with the used space by each user. This column stands near the quota column of course.
This information exists because it is displayed at the top of the personal screen.
Be sure that just this information will help to monitor all given quotas. And if this table have sortable columns, it would be the paradise.
But this feature seems to be closed by Deepdiver https://github.com/owncloud/core/issues/23339
It was closed as a duplicate of the still open issue here: