I don't think you're missing it. I checked Amethyst's source on GitHub (main branch, 22 Oct), and the Notifications tab always includes reactions and reposts of your own notes. There's no in-app switch to hide them.
What the settings do have (Settings > Notifications):
- "Split notifications by Follows" gives you two tabs, Following and Everyone. That cuts the noise but still shows likes and boosts.
- "Show Messages" only controls whether DMs show up there.
- Under Categories, tapping Reactions or Reposts opens Android's own settings for that channel. Turning those off stops the push alerts, but they'll still show in the in-app column.
If you want a conversation-only view, it's worth opening a feature request:
https://github.com/vitorpamplona/amethyst/issues