Duckweed is difficult once it is established.  I dealt with it in 2022 and learned from that.  First, I put in a circulator.  This keeps the water moving and helps the pond digest the muck and nutrients that are feeding the duckweek.  Also, look for a pond herbicide with flumioxazin.  Ultimately, I found that spot treating small areas with 2-4,d on a calm day works as well but treating an entire pond that way would be difficult. The flumioxazin is labeled for it.