We obviously won't have a Welsh Empire after the union with England, except due to a painful break away which is unlikely to let Wales become the dominating power.
Therefore, the PoD must be at about that time or earlier - like the King dies; a civil war erupts about his succession; Wales is unaffected and gets a new King, while England is in ruins; the Spanish are about to come over for a visit; the King of Wales gets the best ship builders and cannon producers from England and the Netherlands to build a small but effective fleet; he's successful in sinking an earlier and smaller Armada, establishing Wales as a major power. Wales expands a little at home, especially to the south, to gain those ressources and the technologies, trade, and taxes associated with such ressources; Wales (maybe together with Scotland) makes sure England breaks up and becomes 3 small Kingdoms not threatening Wales (maybe also Catholic again); Wales becomes Protestant with the next King (has certain economic benefits); Wales gets a few colonies to underpin it's major power status; Wales expands like OTL GB, just a little bit earlier; Wales is the place from which all the English, Scotish and even (thanks to cheap shipping) Irish emmigrants colonize the new world (Americas, Africa, later Australia); The 6 Kingdoms (Wales, North-England, Middle-England, South-England, Scotland, and Ireland) form a loose alliance; South England sometimes competes a litttle, but whenever it's involved in a continental war getting too big, Wales gets the colonies in exchange for soldiers and ships; Cardiff becomes a large city unlike any other in the 6 Kingdoms; the city later becomes famous for her "swimming harbour" (large pontoon piers to offset the tides) and other marvels of Engineering; Ireland becomes part of the Welsh Empire. At the end of the 18th century, the Welsh Empire stretches over a quarter of the World, Welsh is spoken by a third of the worlds population, and there is even talk about the remaining English and Scotish Kingdoms uniting under the Welsh King or Queen.