iPurpl3x the Model extender doesn't allow that I think.
Even if it did, I'm not sure that would fix your issue, because setLastPosterDiscussion() is called with a specific discussion as a parameter already?
What I'd suggest is to register another event listener for the same event, and try to re-update the last discussion after the tags extension does https://github.com/flarum/tags/blob/0862b3ea0ed4be338599dc5d8e28cb6c92f77c5e/src/Listener/UpdateTagMetadata.php
Ideally you could also unregister the default listener to prevent unnecessary database accesses, but I'm not sure whether it's possible to remove an event listener.
It's possible the order the extensions are enabled will cause issues with this approach.
