Yes because he realised his original plan did not work out as imagined e.g. the avengers going back in time to reverse it.

So he decides he will eradicate everything and start a new.

Like Hulk said, past becomes present while explaining time travel

And couldn’t he let Dormammu destroy Earth and save another planet with the same population

True but that's if your traveling within an existing timeline. Thanos planned to wipe the timeline completely and start again

He could but I think it would go against his character wanting univeraal balance. A being like Dormamu would only cause chaos across the universe and I doubt he would stop at earth.

I got it. So he put up restraints?

Possibly, unfortunately there's no official answer

The thing is when Thanos did the snap in Infinity War the Living Tribunal should have actually intervened as it would have caused an imbalance in the universe, with his role being to ensure the universe is in balance.

I guess it would have been to confusing as Marvel hasn't really layed the ground work for cosmic beings yet