Seth is Biblical, and therefore Hebrew. A lot of Biblical, particularly Old Testament, names used to be popular in Wales due to the influence of nonconformism.
(My favourite name of a haulage firm from donkey's years ago - Tyssul Ebenezer. In your face, Norbert Dentressangle!)
Anwen is actually a fairly recently devised name, but it's a combination of the traditional elements An- (which in names is usually an intensifier, so Angharad is from *An-carata = much-loved) and -wen (originally a suffix applied to women's names meaning 'blessed', implying that they were very holy women or saints, so original name Cain (beautiful) + wen = Ceinwen).
Might be worth trawling through lists of Welsh saints' names and Welsh literature - the Pengwern cycle of poems (Heledd, Llywarch Hen), the Gododdin, the medieval tales, etc.
Heledd (dd is a voiced th, the sound at the start of the words 'the' or 'thee') is lovely - a friend of mine brought up a daughter of that name in
SE England without too many problems.
I would love to have called a daughter Ffreuer, after one of Heledd's sisters in the Pengwern cycle, but having an English family-in-law couldn't have coped with her being called Fryer / Friar.