It is my understanding that it is improper to use "would've", while "I've" is fine?


They both seem OK to me.

Have you also considered "I'd have" and "'I'd've"?

Much depends on whether you are asking about speech or about writing, and also on the level of formality that you are considering. Do you want to be more specific about this?

Best wishes, Clive
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I think it's completely fine to say 'would've' (and even I'd've), but we don't usually write it (unless for something very, very informal, like a text message to a close friend).

would've is also okay.