He's coincidentally in the exact right place in all of infinite space to reach the Decepticons that get dumped before they die. He basically takes the opportunity to send them as minions to go kill the current Autobot leader and destroy the Matrix. If he hadn't picked the most treacherous robots in the universe to send it would have worked. Even if they failed, there shouldn't have been a trail back to him to make problems for him.
He doesn't start threatening Cybertron until Galvatron starts rebelling, and it's a sort of "Do what you're told or I'mma eat your planet."
Then Galvatron betrays him, and brings the Matrix back to him. Knowing that Galvatron can't use it, he basically decides to punish him by eating his planet.
The only dumb part is deliberately eating literally the only thing that can kill you, but it wasn't an immediate threat at that point. It only becomes one because the destined Autobot leader just happens to show up in a ship that they crash into his eye. Which gives them the opportunity and motivation to find and use the Matrix in time.
If pretty much anything happened differently in the movie, the good guys lose. If the one group doesn't find the Junkions and save Ultra Magnus, they lose. If the other group gets killed by the Quintessons (or just take too long to escape), they lose. If Hot Rod gets killed by Unicron's internal defenses, gets killed by Galvatron, or just gets lost inside of Unicron/doesn't meet Galvatron, they lose. If Galvatron behaves, they lose. If Unicron just happens to go a different way on his travels, he misses the Decepticons, never sends them, and the good guys never have a chance to stop him. And so on.
It basically takes a ridiculous chain of unlikely events strung together to set up the exact scenario that defeats him.
You're not wrong but he technically alerted the Autobots as soon as he started eating the Cybertron moons. Maybe he legitimately didn't know they were Autobot bases?