@welshbabynames , Wyn is good because it sounds like 'win'.
Wyn was massively overused as a welsh middle name in the mid-20th century, but might seem fresher now.
Hardly anyone will get Geraint right, and although it is a nice name, I wouldn't use it. The cyclist's name was as a pp said truly mangled. Even if they got the sounds right (Gerr like in Gerroff!, int as in pint) they'd stress the wrong syllable.
I wouldn't recommend Arwyn as he'll be Ah-wyn and names tend to get shortened to Baz, Gaz etc (Arz not good), and Arwyn Wyn is out of the question
Guto is lovely and not overused, but he'll probably get called Ghee-toe.
Gethin is fine but he'll get Geffen.
Watch out for names with consecutive vowels, they'll get transposed (see Oisan above). Gwilym is lovely and very now but will be Gwylim or Giwlym.
Gareth is fine but Gareth Wyn sounds very 1960 and if IIRC there was a Gareth Wyn in Pobol y Cwm, played by a young Ioan Gruffudd.
Tecwyn is dreadful, Tegwyn is better but looks like 'white tea'. Tegwen for a girl is pretty though.
Merrion is truly awful, Meirion is nice but nobody will say it or spell it properly (look for threads on pottery/uk travel). Welshified spellings of popular names (Aron, Tomi, Hari, Jac etc) are fine in Wales but look wrong in england. Tomos isn't too bad.
Elias is quite popular in Wales but tends to be said as 'LEE-ass.