Must I re-observe a day of Ramadan due to the devil's whisper that I drank, knowing that I completed the fast with the intention of making it up, and then fasted after a period of time?

The cure for obsessive thoughts (waswas) is to disregard them and pay them no mind. You were not obliged to re-fast that day, as the matter was merely an obsession, a doubt, or an illusion. You must ignore obsessive thoughts in all matters of worship and not repeat ablution, fasting, or prayer unless there is absolute certainty about the existence of something that necessitates repeating it.