Power levels aside... I don't think Chronos can realistically take over Marvel Earth. How would they infiltrate? There are so many superheroes and supervillains, secret governmental organizations, secret supervillain organizations, sorcerors, psychics, telepaths, gods, demons, self-aware supercomputers and androids, terrorist cells, free agents - super-powered or otherwise - who work for nobody, mad scientists, mutates and mutants, not to mention several other sentient species - some of which are not even related to humans - living on Earth, plus at least one strain of sentient microbe which has steered the course of evolution for billions of years, that an operation as vast as Chronos would have a very hard time getting a foot in the door without causing a dozen alarm bells to ring.
"Okay... we have a subject... shall attempt preliminary testing to see what type he could be processed into... any personal data?"
"Um... this is the guy's wallet... let's see... occupation... oh, hey, he works for the DOD's science department."
"I bet he's a Gregole. You can tell by the build. This guy was made to be a big green bruiser."
"Name... Robert Bruce Banner -"
***
"Okay... we have another subject.... shall attempt preliminary testing to see what type he could be processed into... and check his date beforehand this time!"
"Holy crap, there's a million cards from clubs in here... and what's this? A little black book? Hey, I'm keeping that."
"Get on with it! What's the guy's name and occupation?"
"Uh... party-animal, by the looks of things."
"Ha! We'll give him a purpose in life."
"Name...Jonathon Lowell Spencer Storm."
***
"...All right, a woman this time. Attempt... preliminary testing to see...what type she could be processed into..."
"I can't believe we're processing women now."
"What I can't believe is we lost two labs already because the people here are all freaks. Let's just hope this girl doesn't have any nasty surprises..."
"Nahh, she's out like a light. So... who is she?"
"Doreen Green. And - "
"Hey! How did that rat get in? Filthy animals!"
"...That's not a rat, dude. That's a squir-"
"AAAAEEEIIGGHHH..!!!"