I had the same problem as you and finally found this extension: https://extensions.xwiki.org/xwiki/bin/view/Extension/Admin%20Tools%20Application
When you run it, there’s a section called Show Rights which gives a complete overview of all permissions which have been set in your wiki. It’s not perfect, but it is much better than the default permission viewer IMO and I wish I’d found it sooner. Hope this helps!