Best way to organize large amount of resources in navigation

I have a lot of resources (currently 38) in my admin panel. Right now, I have organized them in a couple of NavigationGroups. This works, but its still a bit confusing for my customers, since the sidbar is getting very long and then sometimes don't know in which group the find a specific resource. In the near future, the number of resources will grow.

Any suggestions how to solve this issue? Is there maybe an option/plugin to "filter" the sidebar-navigation? Or can I have another (sub)level of the navigation?
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