Just a thought but have you tried The Somerset and Dorset Family History Society to see if they have any information on this family?
https://sdfhs.org/I searched on Freereg to see what other Pyne (and variants) families were baptising children in Wellington around 1779. You may have done this already but for your information these are the entries:
1776 Betty Pyne, d/o John and Betty Pyne.
1778, Sarah Pyne, d/o John and Elizabeth Pyne.
1778, Hannah Pyne, base born daughter of Sarah Pyne.
06/01/1783, Sarah Pyne, d/o John and Elizabeth Pyne.
06/01/1783 Mary Pyne, d/o John and Elizabeth Pyne.
1785 John Pyne, s/o Edmund and Sarah Pyne. In South St Baptist Chapel.
1787 Nathaniel Pyne, s/o Edmund and Sarah Pyne. In South St Baptist Chapel.
1787 Sarah Pyne, d/o Joseph and Sarah Pyne. In Presbyterian Independent.
1789 Betty Pyne, d/o Edmund and Sarah Pyne. In South St Baptist Chapel.
There is an odd baptism for a Sarah Pyne, baptised in Wellington on 16/09/1833, born 10/05/1776. Daughter of the late Henry and Mary Pyne. Father's occupation: Woolcomber. Mary shown as wife of Thomas Baker.
There are some Pyne baptisms in West Buckland, which is close to Wellington. These are as follows:
Nathaniel Pyne, born 1773, baptised 02/04/1782, s/o John and Mary Pyne.
John Pyne, born 1775, baptised 02/04/1782, s/o John and Mary Pyne.
Robert Pyne, born 1779, baptised 02/04/1782, s/o John and Mary Pyne.
Charles Robert Pyne, born 1776, baptised 02/04/1782, s/o John and Mary Pyne.
William Pyne, born 1778, baptised 02/04/1782, s/o John and Mary Pyne.
Mary Ann Pyne, born 1781, baptised 02/04/1782, d/o John and Mary Pyne.
Henry Pyne, born 02/08/1782, baptised 17/09/1786, s/o John and Mary Pyne.
Thomas Pyne, born 07/02/1785, baptised 17/09/1786, s/o John and Mary Pyne.
There seems to have been a tendency for the Pyne families to have children baptised in job lots some time after they were born. I have seen other examples where Pyne children were baptised years after they were born. Another example in Wellington, St John, Elizabeth Pyne, born 1790, baptised 06/07/1806, d/o James and Mary Pyne. One more odd one: Lucy Payne, born 17/09/1790, baptised 14/09/1819, d/o Peter and the late Priscilla Payne. Father a Gentleman.
It might be worth bearing in mind that Issac Pyne might have been baptised a long time after 1779.
I presume that all these Pyne families in that area would have been related to each other.
Jo