Fortunately, there's a workaround for that. Originally I had this whole convoluted approach that involved scheduling the last-prompted-for-upgrade date to be today's date. Thanks to a tip from @gmarnin (on the Mac admins Slack), apparently you can set the date to be in the future, and it shouldn't prompt you.
I have created a profile for that, which you can use if you'd like. Just sub in the actual name of your organization for NAMEOFYOURORGANIZATION.