Ok, so where does the toast go if you've already scrolled or otherwise navigated to a different area of the UI? These optimistic updated could take multiple seconds to succeed, and maybe as much as 30 seconds to fail.
Not to mention that, if the operation fails, isn't it likely that the user will want to re-try? And that'll require access to the original control in all likelihood.
If the user scrolls away and the information is important, use a modal alert.
These toast notifications just are a bad solution. I often miss them, because I'm, you know, doing work, not scanning my monitors for notifications I mostly don't care about. (Redundancy is not harmless. Redundancy also trains me that your messaging is mostly noise.)