I don’t think!

What is the best Greek phrase or words for this very common English idiom in slang, when added to an ironical statement? Here are a couple of examples:-
-Person A: “She is a woman of low morals.”
-Person B: “You are the paragon of virtue, I don’t think!”
-Person A: "I think I'll use my phone while driving to find the quickest route."
-Person B: "Yeah, great idea, I don't think!"
-Person A: "I'm going to hand in this paper with no name on it."
-Person B: "Wow, brilliant move, I don't think!"

Could you use ναι καλά or αιντε! or σιγά? But these don’t sound appropriate equivalents,

Is this like the americanism that was quite common up until recently where they would make a statement and then say "not"?

No SBE. Apparently, it originated in the late 19th century, particularly within British Cockney slang and literature, becoming highly popularized in the late Victorian and early Edwardian.eras It was commonly used in London in the 1880s and 1890s to negate a positive statement that was intentionally exaggerated or sarcastically meant. The use of the irritating slang Americanism you refer to is frequently linked to 1980s Southern Californian teenager slang ("Valspeak"), which was then popularised nationally in 1982 by Saturday Night Live and films like Wayne's World.

May be "τι να σου πω, δηλαδή!"