Hello cloudcuckoo, welcome to the thread. Your wife is the third of Septimus' great-grandchildren to tangle with it (the other two are MaggiMae through Nellie's brother Henry, and me through Nellie's brother John / Jack). It seems that all of Septimus' children had to copy out the journal of their grandfather's trip to America!
As far as the Pierce ancestors in the 18th century are concerned, the tradition that they came from Wales and the West Country now has names and dates, although I am not sure if anyone has verified them (much of what I was told about the Pierces has turned out to be hogwash, so we need to cautious). Not that we really need a Welsh immigrant to explain George's appearance in St. James Westminster in 1769, since there were multiple families of Pierces living in the parish in the 1730s and 1740s, and he could have been descended from any one of them.
Do you have any old Pierce family photos, documents or memorabilia, apart from the copy of Samuel Pierce's journal? We have a few interesting things that we would be interested in sharing. Also a little information about the maternal lines (Sophia Brorone/Brown & Ann Margaret Morgan). If you make 3 or more posts you will be eligible to send and receive private messages, which may be preferable for this sort of exchange.