Your premise is flawed. Not because parallel universes don't exist - they're how time travel is achieved in the MCU - but because Thanos does change his plan.
Specifically, at the same time that he learns about time travel and parallel universes, he learns that the Avengers are planning to use said time travel to undo his success in the original timeline. This causes Thanos to realise that, as long as the universe remembers those they lost in the Snap, they will never be grateful towards him for doing it, so he revises his plan to "destroy the universe and rebuild it from scratch so that it was perfectly balanced from the start".
In other words, Thanos changes his plan from killing half the universe to achieve balance, to resetting the entire universe to achieve balance. And if he's already achieved balance, then he won't need to go raiding other timelines.