I think you can customize the individual preview texts as was suggested in Change in behaviour re mail - #11 by supermathie.
You can find more about editing texts in Customize text in Discourse
So, for example, to prevent “Someone replied to a topic you are Watching,” you would remove the site text in user_notifications.user_posted.preview. You cannot leave it empty, but you can use an invisible character like ㅤ.
You can see the difference here:
You can do the same with all the other preview texts.