Good evening, @barsmith ...
I think your workflow to create a new assignment based on an existing assignment could use a little tweaking. Rather than copying/pasting content from the existing assignment to a brand new assignment, have you instead tried using the "Duplicate" feature? This simply creates an identical copy of your assignment in the same course that you can then edit as needed. See How do I duplicate an assignment? for more details.
I hope this will be of some help to you. Keep us posted here in the Community if that solves the issue where things are freezing up for you.
Take care, stay safe, and be well.
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