Hi Ian
I know it is frustrating and discouraging but don't lose heart yet
You could well be right about the wheelwright evolving into a carpenter. Don't think they would have married after having several children. The early children would have been noted as being base born otherwise. In which case what you have are the following:
1769 Edward
1771 John
1773 Robert
1776 Jane
1778 Godfrey
1780 Robert #2
1782 Thomas
1784 Jane #2
1787 David
There are two burials in the Llandegla registers which don't fit the above dates at all. Both Janes appear to have been buried before they were baptised - perhaps the wrong registers were used to record the events
Dec 19, 1774 Jane ROBERTS, d/o Robert (carpenter) & Mary
July 22, 1783 Jane ROBERTS, d/o Robert (carpenter)
There is also a Robard Robards s/o Robard & Mary baptized Oct 4, 1767. Its possible that the baptism for the Robert in 1773 was actually a burial but that would disrupt the pattern which they seem to have established!
The deaths of 1788 and 1798 for Robert & Mary make sense. Too bad there are no ages noted. So where to find the marriage of Robert & Mary? Have found one
possible in Llanelidan Jan 21, 1766:
ROBERTS, Robert otp, Ba, Licence
ROBERTS, Mary otp, Sp,
witnesses Hugh Jones and Robt Goodman
I looked for the marriage bond/licence on NLW but it does not appear to be there. You could email them to ask about it as they may have more that have not been listed yet. If I were you I would also do a search on the North Wales Marriage Index - either through the Clwyd FHS or through Genfair(on line), asking for Robert ROBERTS and Mary marriages. It might narrow the field down a bit - if there is only one nearby in the mid 1760s the Llanelidan couple are probably your best bet.
Heather