Hello osprey
Going back to the possible family of my ancestor William Bigham who married in Llanishen in 1776 -
His son William (bapt. 26 Dec 1776 Llanishen) married an Ann Bigham on 30 Mar 1809 in Bristol. I found them on the 1851 census which lists Ann's birth place as Llansoy and I found her death in 1854 which lists her age as 66, making a rough year of birth as 1788, which corresponds quite well with the age listed for her on the census. I then found what I am quite certain is her baptism on 23 Dec 1789 in Llansoy, daughter of Robert and Sarah. I also found baptisms of four older siblings.
Her parents Robert and Sarah nee Cadogan married on 19 June 1771 in Llansoy. Robert was listed as being a bachelor of Llanishen. This ties in with him being the Robert Bigham whose baptism you found on 2 May 1744 in Llangwm, son of Robert and Mary, who you found later also had sons baptised in Llanishen. I also found Robert and Mary's marriage - Robert Bigham and Mary Williams on 2 June 1742 in Llangwm. Robert was listed as being of Newchurch, as he was on their son Robert's baptism (and the baptism of their next son John/Benjamin in 1746 was in Newchurch).
I believe Robert and Mary Bigham nee Williams may have also been the parents of my William Bigham, as you also mentioned the possibility of in your message. Their son Richard baptised in 1751 in Llanishen is probably the Richard Bigham, bachelor of Llanishen who married Elizabeth Lawrence, widow of Llangoven by licence on 18 Dec 1773 at Llangoven and had at least two children baptised in Llangoven.
They may also have been the parents of the John Bigham, bachelor who married Elizabeth Walters, spinster by banns on 4 May 1775 in Llanishen, both of Llanishen. Robert and Mary's son baptised on 11 May 1746 in Newchurch may have been named John as stated on Freereg. Or he may have been named Benjamin as you said it looks like on the register.
I viewed an image of the register on Findmypast and it looks like the edge of one page overlaps another page, so parts of names, including John/Benjamin's name, were cut off. So, on the left hand edge of one page you can read the letters am, and then on the overlapping page the entry continues as "in the son of Robert Bigham was baptised the eleventh day (of) May (the word 'of' is also cut off). So the only part of the name visible is amin, which makes it look like it was Benjamin. However, I haven't found a marriage or burial of a Benjamin Bigham yet, but maybe he died young. If he was named John instead then he could be the John who married Elizabeth Walters.
They had three children (John, Ann and William) baptised in Llangoven in 1776, 1780 and 1783, but there were also children of a John Bigham named William and Elizabeth baptised in Llangovan in 1774 and 1775 respectively, however I am thinking their father must have been a different John Bigham, as the daughter Elizabeth was baptised on 24 Apr 1775, less than two weeks before the marriage of John Bigham and Elizabeth Walters in Llanishen. And the William baptised in 1774 had the same name as William baptised in 1783, however I have not found a burial for the first William before the second William's baptism date.
Just need to connect my William Bigham and possibly also John Bigham as sons of Robert and Mary or find out if William and/or John were sons of another Bigham couple. Robert and Mary's son Robert's baptism date was 23 months after their marriage date, so they could have had another child born before him. Maybe that child was William or John. As you say, there were also gaps between the births/baptisms of John/Benjamin in 1746 and Richard in 1751 and between Thomas in 1754 and Jonas in 1758, even possibly between Richard in 1751 and Thomas in 1754 also.