Hi Roger,
I've never been able to find Strygosfawr or Twyn-y-rhose either. Looking again at the 1769 Doc. it says, "messuages and land". Messuages are the house and other buildings so there must have been an actual place, rather than the doc being simply a transfer of land. Lledrod is a large spread-out parish but there isn't a peep of the properties. Looking at another doc. regarding the sale of the mill at Lledrod, we have 3 names for the same house, "Ty Issa, or Wainhelig, or Wainbaby" so the name Strygos may have changed, concealing its' location.
My notes show that David Richards was the resident at Ffosybleiddiad from 1757 to 1843 and then his son, David Meredith Richards was there from 1843 to C1870's. In 1880 the place was sold by the Lloyds of Mabws to the Trawsgoed Estate. I also have note of various servants at the house and a note that the huge kitchen of the house was used to house students at the Edward Richard School. (They slept in hammocks strung up in the rafters).
The dates that you have for Bryn Issaf are too late. John and James first appeared in Llanerchymedd in the early 1790's. their parents would have been born C1735/40.
I'm now looking at Llechwedd, Llangwyryfon, where there is a nest of Richards. The "Head" is a David Richards, (1801) with a wife Mary and son Daniel. Fingers croseed.