What's the likelihood that stopping system updates could prevent the disabling of sideloading capabilities on android this September?
Depends. The update will likely be rolled out via Google Play system updates and not a usual Samsung One UI update. Disabling Google Play Services might also be enough to restore the functionality of installing ("sideloading" is a misleading term, making it seem like installing software from other sources is unusual, weird, and "shady").
I also highly recommend against stopping OneUI system updates, since you will miss out on important Android security patches.