'Preachy', I believe, has a reputable history; 'proudy' does not (check out most of those googled results!). Adding '-y' can happen ad hoc and casually, but discovering whether a particular word is acceptably standard will require a dictionary search.
"Proudy" is not a word. (Except, according to UrbanDictionary.com, as a slang noun referring to a certain type of man. I've never seen or heard it before.)