Pretty darn impossible.
You will need a literate (are there some?) and sober (there may be one) Welshman to CORRECTLY transcribe a written language.
This language makes the Irish Kelt look literate.
I believe the most recent Englishman trained in both languages is a prince and assigned to other duties.
The problem is much greater than phonics.
There is nothing "wonderful" or "secret" to be discovered.
Spend you time, singing, and drinking, and playing what you already know.