Is the fasting of David to be combined with fasting on the monthly, weekly, and annual meritorious days, taking into consideration the saying of the Prophet, peace and blessings be upon him, concerning the fasting of David: "There is nothing more virtuous than that"?

You are upon good, whether you fast for one day or the entire month of Sha'ban. The best is to do what you find spiritual energy and pleasure in, without boredom. It has been mentioned that it is better for someone who fasts one day and breaks fast another, if their breaking of fast day coincides with a Monday or Thursday, to fast on that day due to its virtue, and this does not deviate from the fast of Dawud.