Is the Ash'ari doctrine that the Qur'an is ancient, not created or originated, and that its letters are ancient, considered an innovation (bid'ah) or a deviance (dalalah), or what is its ruling in Islamic Sharia?

What you have mentioned, that Allah speaks by His will, is the belief of Ahl al-Sunnah wal-Jama'ah (the People of the Sunnah and the Community), supported by texts from both revelations. Sheikh al-Islam Ibn Taymiyyah said: "Indeed, the speech of Allah is an attribute of both His essence and His action; He speaks by His will and power, and His speech subsists in His essence. This is the understandable meaning of the attribute of speech for any speaker. The Qur'an is the uncreated speech of Allah, and Allah speaks whenever He wills, with speech following speech." And whoever among the Imams said that speech is ancient, his intention was that it is uncreated, not a denial of the renewal of speech. We advise to stay away from obsessive thoughts (waswasah).