Regardless of if the Welsh, Irish, and/or Basque all ever actually stepped foot on the Americas, they most definitely fished in the Grand Banks, there is no dispute about that. That's fact. They almost certainly knew land was nearby, MAY have even occasionally landed. But they didnt care. The technology to reach the Americas existed far too long ago for the idea that "we'll just delay the technology". The Scandinavians were making it there over 500 years before Columbus, and before even Vinland, because Greenland counts as the Americas, hate to tell you about this thing called geography. Eventually some Welsh or Irish or Breton or Basque spills the beans to a competitor, who encourages England, France, or whatever is in Spain "hey, give ME the monopoly over this great fishing area that also has all these trees on this unofficial land! We'll get rich off selling fish during lent!" it's not gold, G-d, and spices, but it's a beginning and probably happens in early 1500s. Any POD that specifically slows down Columbus et al may have unintended consequences of spurring other nations to go even earlier than Columbus (the post above about less trees in the the rest of Europe drives up the price of fish and trees and makes exploration to the west or south to Africa even more appealing). I see OTL as the most delayed possible to be honest. I don't think much more than what already happened in OTL could do any worse.