Is it permissible for a person to adopt a daily litany (wird) of seeking forgiveness for themselves, obligating themselves to a minimum amount of it?

There is no objection to a Muslim taking a specific daily litany of seeking forgiveness, or other invocations, and Quran recitation, and maintaining it daily. This is the Sunnah of the Messenger of Allah, peace and blessings be upon him, and of the righteous. He is not obligated to perform it as a necessity unless he has made a vow to do so.