Why start with him?
Several answers. He has been divined to have known Y-DNA, the only ancestor of whom this must be true, by the same DNA being inherited by two descendants alive today, one each of the only two male lines from him. Further he was a distinguished person in his own right, a feat not inherited in the majority of his descendants. His ancestors are very difficult to research as my brother Martin found when he did his excellent research on them. For all his children it is possible to find all their baptism records most of which included their birth dates, marriage records are almost as easy to find though some of the deaths cannot be found, particularly for those who died young. And the records for his children's children are similarly to be found.
Might this become a definitive record?
My ambition is to create a definitive record of the births, marriages and deaths of all his descendants. This record should include all records of those events that are not penally expensive, such as all births, marriages and deaths from the registration system which could easily require 3 certificates for each of 1,100 persons at £11 a pop: you can do the arithmetic to calculate the total cost. Of course I am failing in this objective: some of these records I cannot find. But I hope that a future family reasearcher will continue the search as more records become available around the globe.
Another feature is that of including a transcription of the records that I can find. The privilege of doing genealogy in these times is that so many copies of the original records are, in England and Wales, available for us to read. The copies spectacularly improve the accuracy of the resulting genealogy. The problem is that each copy of a record is an image file averaging around 2 Megabytes per copy. This is a lot. For the senior line of Powys descendants I have so far accumulated some 1000 images. So it would need 2 Gigabytes of web storage to hold them all and never mind the slowing down by all that data being transferred to any reader's workstation. So I have made as accurate a transcription as possible perhaps consuming anything from 100 to 1000 bytes per record, radically reducing the storage space needed and likewise speeding up the transmission to the reader.
Female descendants are equally descendants
A difference is that in these times of equality I must include all sexes. Roughly, adding in the females must double the number of descendants to be studied. Further the females do not, mostly, sport the surname of Powys; their married name is transmitted to their children and it can be difficult tracing who begat whom as a result. From the later 20th century there are few censuses to confirm the family groups. And in the 21st century the marriage records effectively vanish from 2006 onwards. Our only salvation is that wills are more habitual than in earlier centuries and very easy to get hold of in England and Wales and many people like to name their relatives in their wills and leave them a token for remembrance.
It could take forever to complete
I did some sums recently on where I had got to with this study. Of course I do not know whom I have yet to find but I have in the past found, by less reliable searches, a lot of descendants and have been blessed by those who have got in touch to tell me more. I have put together a chart (click on 'chart', then click on the chart's 'back' button to return here) of where I think I have got to with these descendants. You can see that I have estimated that I have at least 330 of around 1500 known and unknown descendants still to do. This could easily take six months, particularly as I have come to a halt while I update all my web-site's genealogy. This would be the first pass. Then there are second and third passes where I uncover omissions and infelicities. I can't see that I'll finish this Thomas-Powys-Descendants project before the end of 2022. But those that are said to be Done in the chart are indeed on this update of our genealogy database. Others 'Not Done' may also be in the genealogy files but I have yet to examine them to discover what documents are available to confirm or provide the data about these people.