Hi @cduke ...
I see what you mean ... using the screen via the "Assignments" index page to batch edit assignment dates. I know this might not be the exact solution you're looking for, but @James has a nifty script that you can use to change assignment due dates via a Google spreadsheet. His method was developed long before Canvas came out with their own solution. So, you might want to give it a try. It's called a "Canvancement" (Canvas enhancement):
Adjust All Assignment Dates on One Page - Instructure Community (canvaslms.com)
He has a full description of what it is, how it works, and also some video instructions on how to use it in your own course(s).
Give it a shot...and if you have questions, you can post those at the bottom of his write-up so that he can respond.
Hope this will help a bit. Take care, and be well.