Welcome to the Asana Community Forum @Rebecca_Scott1!
I agree with Christine in regards to archiving old projects.
I‘d probably even also move all old projects into a separate team where you can store all archived projects. Then you can start fresh since you mentioned you will set everything up from scratch.
Now in regards to tasks I have been seeing different comments here on the forum, some just mark the task as complete, others move them to an archived project, etc.
Here is an idea I recommend as this will allow you to still filter completed tasks that weren‘t actually completed.