I dunno man, “a madman thinks he understand the universe way more than he really does” doesn’t break suspension of disbelief for me. I agree with you that his plans are flawed, but he’s not infallible. I’m not trying to gage whether or not his plan makes sense, I’m trying to gage whether or not him believing it makes sense. History is full of arrogant men with half-baked plans for salvation; I don’t see how Thanos is any different.
To be clear though, I also agree that the comic book version of Thanos’s motivation was way better. Like not even a competition. But I don’t think the MCU version is so nonsensical so as to be unbelievable as a motivation.