A few other points that seem to me to make more sense of some of the data,but don't necessarily
lead to the answer(s) you want yet.
In the Crosswood2(B) documents at NLW there is a reference in 1719 to a David Richard,yeoman of Ynis Pervedd(sic) with an interest in Tythyn Bryn Merllyd with a son and heir Richard David.As we know the property is at least in part synonymous with Tythyn y Bryn Issa and later on apparently Pen(Y)Bryn.There are three relatively adjacent properties in Swyddffynnon on current maps- Ynys y Berfedd,PenBryn and Bryn Isaf and not too far from Ffos.
The will of 1722 of David Richard which I quoted may not be relevant.There is a will of 1727 of Richard David of Ystrad Meurig who also,it seems,has a wife Magdalen(a little unclear),and inevitably,amongst others,sons David and Edward.
The David Richards(note Richards) of Lledrod first seen I think in these documents in 1742 is given as a gent-perhaps too early for the "conversion" you speak of.He seems to have dealings with a Rees Parry,who has a wife Letitia.Perhaps he also has a daughter Letitia and it is she who marries David Richards in 1737.The 1742 interest is a property called Henrde Rys.
There is not an obvious known family link between David Richards,gent, and the yeoman's family,yet in 1753 there is David Richards of Lledrod,gent,with an interest in Tyddyn y Bryn Merllyd!
Regards
Roger