Hmmm, just spent another moment looking at Digby BMDs in Essex 1715-1740 on FreeReg.
I think your James and Sarah who baptised a James in May 1722 had no other children baptised in the same area.
It's quite possible that they baptised James in 1722 (though he isn't mentioned in the transcription as being a base child) then married in 1727 and had the rest of the family (William in 1729, a James in 1730, Joseph in 1732, John in 1734 and Benjamin in 1738).
The problem is the second James in 1730 - his first name would imply that the James born 1722 had died - but I can't see a burial on FreeReg for this earlier James. Haven't looked elsewhere.
I think this must be a completely different James and Sarah, the parents of the family in brackets above. If their first child was William born 2 years after their marriage then it's likely that William was the name of the paternal grandfather, which might be traceable.
The second James born 1730 would only have been 16 if he is the one who married Margaret in 1746 - but in any case there is a burial of an infant James in 1733....
Looks like a great deal more crosschecking is required !