I don't know why I didn't think of this before,but given the 1757 reference to Bryn Issa,and your statement of the births there of John and James in the early 1760s,I thought I would look for any references in Crosswood 2(B) after this date.
The following looks interesting:-
1777, Jan. 17
INDENTURE, being a declaration by Morgan Harry of Fronfelen, p. Llanychaiarne, co. Card., gent., and Letitia Richards of p. Llanygrwyddon, co. Card., widow, that a fine had been levied at the Great Sessions for co. Card. on 14 Aug. 1762 by Elizabeth Richards, widow, the said Letitia Richards, Thomas Richards, gent. and Jane his wife, of two tmtís called Penbryn-issa alias Tythyn-y-bryn-merllyd, and Bryn-merllyd alias Tythyn-
bryn-issa, and of a parcel of land called Gorse-glan-teify (60 a.), p. Lledrod, co. Card., to the use (as to the said parcel of land) of the said Morgan Harry in fee, but that no uses had been declared as to the said two tmtís, they therefore declared that the said fine should enure (as to the said two tmtís) to the use of William Davies of Penbrynn, p. Llanvihangel Lledrod, co. Card., gent., in fee simple. It recites that the said Elizabeth Richards, Thomas Richards and Jane his wife had all dec., and that the said two tmtís were the inheritance on the said William Davies.
I'm not sure if the Thomas Richards is the same as the 1757 one and I have not tracked William Davies.
However,elsewhere Thomas Richards is described as the second son of Rev Morgan Richards,clerk of Radnor whose will of 1749 is available at NLW.Given the date of his death this could give a birth date as early as,say 1710(?) for Thomas,not impossible as the father of James and John.
Not direct evidence,but a daughter of Morgan Richards(as the patronymic doesn't seem to be used(?) is Rachel Richards.There is a marriage to a Charles Price in 1737 which might fit and Charles Price(("Pryse"),perhaps as father and son,feature throughout this period with familiar associations
-Swyddffynnon Mill and William Davies.Perhaps this is the father:-
1697, Sept. 20
INDENTURE, being a further charge of £30 from David Richard of p. Lledrod, co. Card., yeoman, and Goley his wife, to Charles Pryse
gent., on the moiety or half of three parts of an ancient tmt called Tythyn-bryn-merllyd in gr. Mevenith, and formerly parcel of the dissolved abbey of Strata Florida.
All a bit rambling and tentative,but it feels like the answer is somewhere in here.