Ego's plan would have worked if he wasn't such a moron and knew when to keep quiet

First off, even if Peter found out the truth later, that would have been after Ego had already carried out his plan. Peter finding out later wouldn't have hurt his plan after it already succeeded. Telling Peter right at that moment destroyed Ego's plan. That's what made him so stupid.

Second, how the hell would Peter have found out later that Ego gave his mom cancer? It's not as if he had stray paper documents or computer files proving it just lying around his planet for Peter to find. The only proof of what Ego did is what he told Peter. Without Ego telling Peter what he did, Peter couldn't have possibly ever known. People get cancer and die all the time due to entirely natural causes. Peter always assumed that was the case with his mom and he never would have had any reason to even think otherwise without Ego going out of his way to tell Peter what he did.

Ego may have been around for centuries but he obviously understood the concept of love. He loved Meredith and so he must have understood that Peter loved her as well. He was smart enough to understand what it would mean to Peter if he told him that he caused her to die.

You're listing some pretty superficial stuff, some of which isn't completely accurate. Thanos didn't start as an enemy of the Guardians of the Galaxy. He first appeared in the films as the enemy of the Avengers, but was already off doing bad things even before that. Ego never seemed to be taking the good of the universe into consideration. He was killing indiscriminately in order to further his own selfish needs, and never expressed any sense of altruism or duty.

In the ways that seem to me to matter most-- motivation, personality, behavior, history-- the two are completely different.