What are the best miles cards for big purchases?

If you have a big one-time expenditure coming up, it's highly recommended you look at sign up bonuses to get the most miles out of it. A full listing can be found here: https://milelion.com/credit-cards/current-credit-card-signup-bonuses/

Check the terms of the card to ensure that the MCC of the hospital bill does not fall under the exclusion list.

From my knowledge, Citi PremierMiles is excluded.

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Was going to suggest the Amex Krisflyer coz they have nice sign on bonus running (and hospitals generally ok with accepting Amex)

Given you won't be eligible since have card alreaady - suggest just look for any card that gives signon bonus; the bonus would be much bigger than regular earning rate for any other card and given your large lumpsum expenditure coming you can easily attain any spending requirement.