O’n and bo’fi do indeed feature in SSiW slightly later on as shortened (slightly rapid) spoken forms. They are a bit more like “I’d” in form, than gotta, but yes, they are very common in speech.
So yes, you would see them in a text message, social media post, friendly e-mail etc. Also in reported speech in a book, but not so much in a government letter or similar.
You are correct about “Tua”. It is one of the words that doesn’t get mutated in Welsh grammar