There is a Marriage 07 Dec 1780, St Michael, Rudbaxton;
Sarah LLEWHELLIN/John THOMAS
Witnesses Thomas DAVIES/Thomas LLEWHELIN
Bride & Groom signed with mark 'X'
(FreeREG)
I don't have access to bapts but there are a couple of JAMES THomas's born c 1786 +/- 5 years on Census who could be marriage witness and possible brother of Dorothy.