Zabbix should be able to create a visual representation of item/LLD rule/item prototype/... custom update intervals. Would be perfect if the same visual approach was used to modify custom intervals in a user-friendly way.

In my opinion, calendar-like representation is an obvious choice. It should support Day/Week/Month view mode and moving back and forth in time. Flexible intervals can be visualized using different color/pattern fill. Scheduling intervals can be visualized using different color/shape markers.