Wait for it
I'll have to go with Astinus on the mentioning Ash part. When you mention him, then that means you're going to have your story set in anime canon, which is quite different from game canon. You can still have your story set long after Ash's journey at Isshu/Unova is finished, but if Ash won't be referenced or be featured, it's best not to mentioned Ash's disappearance.
Other than that, very interesting premise. Don't think I seen the deadly sins being used in Pokemon, so it's would be very interesting how that will play out. If you can't think of some fakemon for the deadly sins, you can maybe still have some of the legendaries be them. I mean, if going by anime canon, the Orange Islands have their own myths of Lugia different from the people of Johto, so it's possible.