Just to sum up what we do know, your Bellamy line is:
George born 1773
George (1749-1804) and Mary Burrow (1749-1783)
George (1713-1798) married to Mary Winnefred Forster (1712-1789)
Josias Bellamy (dates unkown) married to Gathred Mitchell (dates uknown)
Per the 1861 census,
Elizabeth Winifred Bellamy 1846-1929
(E Winifred Bellamy who, or whose estate, sold the George Somers portrait) was the daughter of
Peter Franklin Bellamy, baptised Plymouth 1804, and Mary A
https://familysearch.org/pal:/MM9.1.1/KC9J-MH8So Peter must have been a son of George 1773 (and Peter is or isn't your own ancestor, you don't have to tell us - if he isn't, he had a brother who was, presumably).
I see at FS there were quite a few Somers/Summers Bellamys ... including some who wandered off to Brazil.

I had missed your reference to Cornwall re Josias - what is that connection? Cornwall is way easier to research than Devon, although before 1700 things may get spotty. The earliest Bellamy baptism showing at the OPC site is 1707:
http://www.cornwall-opc-database.org/search-database/baptisms/It does show 3 Bellamey baptisms (use just BELLAM to search the databases) in the early 1600s, and a couple of Bellamey/Bellamee marriages and Bellamy/Bellamye burials around the same time.