Separate fleeting notes, permanent notes, and project notes. Confusing them together creates mess.
Put project notes in a separate slip-box. Delete (or archive) it when project is done.
Project notes can contain random notes, todos, tasks, etc. These are not ideas and creates mess if put in the main slip-box. (Zettelkasten is about developing ideas, not collecting them)
I can also Treat random notes as a separate “project”.