I am also descended from Richard and Mary. But I cant find a Henry in my tree which was compiled by a cousin about forty years ago. The Harms and the Spooners intermarried a lot in the eighteenth century.
I am decended from Richard's son Joseph. (b 1815)and I have information about his brother Benjamin's descendants. None is called Henry.
Incidentally, according to my tree which was compiled before the internet, Richard's father was John Harms (1742 to 1817)married Catherine Woods in 1763 and his father was Robert Harms (1704 to 1765) married Elizabeth Lee on 1734. And his father was John Harms married to Ann. I have no dates for him but both John and Robert also appear as Harm.
Re the German/British question, I suspect the name was common in both countries but has become rare over here.