Just adding Wales is boring. Cornwall is more of a challenge. Plus, I think devolved Cornwall is inevitable so there's a bit of future-proofing. AH.com in-jokes notwithstanding.
Not sure exactly what you meant (which white fimbriation, and total vs. partial) so here are my best guesses about your suggestion:
The first image (full yellow) would retain the full blue field, but would "take away" the crucial white of England, and Cornwall is no longer there.
The second and third images (partial yellow) have full representation, but they show why I avoided using St. David's Cross. It's not that well-known (compared to the official Welsh flag) and the yellow is quite jarring, so it will end up looking unbalanced and substantially changes the main cross. It doesn't look like a combination of St. George and other crosses anymore, it blends into something new that is more evocative of Germany or Belgium to the average viewer ("we've been taken over by those bureaucrats in Brussels!!").
The green from the current Welsh flag is more well-known, so while it results in a substantial change to the field, at least that change is recognisable as "Wales" to the average viewer. Plus, dark green is relatively similar to dark blue (ask the Japanese), so that combination will look rather "smooth" and equal, while yellow appears more "powerful" than white, so that combination will look jarring and dissonant.
The thin St. Piran's cross (I deliberately made it very thin) behind St. George's cross doesn't seem to substantially change the St. George's cross to me. The white between the red and black visually separate them, whereas the yellow of St. David's cross visually connect them together. Also I feel that maybe years of seeing black outlines in illustrations and cartoons have trained our brains to ignore thin black lines as simply "framing" rather than content.
