"At" suggests that the expert is very skilled at doing a specific activity. "In/on" is commonly used when someone knows a lot about a particular subject.
Both are correct. However, there is a subtle difference in meaning and usage. "On" suggests that the emphasis is on the specific subject. "In" makes the emphasis on the person who has expertise.
"Congrats" is just an informal and shortened version of the plural noun "congratulations". Both have the same meaning and are commonly used. However, "congratulations" is definitively more formal.