"Husband-to-be" or fiancé. There's no such thing as a "to-be-husband".
A "would-be-husband" is someone who would like to be your husband, but you haven't made a commitment, and perhaps would never consider marrying him. (This applies to a "would-be" anything. It only depends on the wishes of the person who "would be" something, and perhaps no one else agrees.)
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