I don’t know the answer here but I would incline toward the idea that it’s actually a racial rather than merely cultural grouping. All those Celts rather than Anglo Saxons etc. Of course, thinking this way does mean that South Wales isn’t Welsh but then that’s true anyway. Be a slur on the hell that it is to call it Welsh.
But we do still get this:
This week, Gwynedd council raised concerns that Welsh people of colour will have no tick-box option to indicate they are Welsh and minority ethnic in the 2021 census, as a result of the survey format designed by the Office for National Statistics. This is a bitterly disappointing and frankly astounding omission. It implicitly embeds the dangerous assumption that people of colour aren’t Welsh, or that those who can speak Welsh have to be white.
So, what is Welsh? A culture or a race?