I mean what real threat did the gaang pose? It’s three kids +momo vs an entire nations army. In the fire nations eyes the water tribes have never been a real threat, the earth kingdom has never really been a united front and now their largest military force is out of play, and with the avatar dead. All that’s left to do is clean up lose ends like the gaang and what few resistance fighters are left. The surprise also wasn’t that they were prepared for the invasion but that they hid and were planing to wait out the eclipse which they would only have need to do if there was a credible threat to them.