Once Thanos comes to know about parallel universe after Nebula is caught , which means there is infinite amount of resources in this world. Which can be shared and current population can continue to survive.
What still motivates him to kill half the universe? His point was to maintain balance.
Now we just move resources across time lines and maintain the balance.
Why he continues this motive?
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.
Thanos did not find out about parallel universe, only about time travel. Time travel would not solve the problem of finite resources in the universe.
Also stealing resources would make people in other parallel universes starve, so it would be against his motive to help some people while making other suffer.
Infinite resources mean infinite amount of being that want to deplete them. And because as Thanos know there are more and more beings who want to use them. So infinite resources but "eaters" is infinite plus 1 going to infinity.
Borrowing through time is doing nothing because If you have two of things when you start using them and then go back to BEFORE you started using them to borrow for future then you don't have two anymore. Because you depleted your resources earlier. Hence forbidding you to even start to use those hence not creating need to go back to past to borrow them hence not creating lack of them hence allowing you to start using them hence creating need to go back (and so on ad infinitum). Welcome to Time travel 101.
