About to prep this campaign. Help!

Hello there! My party has just finished PBtSO and Vecna is our next game. The original plan was for them to continue on with their current characters, who started at level 3 and played the events of Nest of the Eldritch Eye, but they are now saying they want to play new characters which is totally fine. I’ve also been seeing a lot of content about “fixing” the module and to an extent I agree. I don’t like the Kass twist and will probably keep Mordenkainan as the real deal. I would also like for the PCs to have a connection to Vecna and have him be a present force, not some villain you meet for the first time at the final battle. My current idea is as follows:

My group in previous campaigns and one-shots have set Phandalin up to be an adventurers hub of sorts. I’m thinking one day they are going about their business as usual. Sleep and wake up to The world having changed for the worst in an instant (think Back to the Future 2) where Vecna has won and the “Doomed Forgotten Realms” reality has taken over. This would mean that Phandalin has become a Mind Flayer colony, Baldurs Gate has joined Elturel in Avernus and all the bad outcomes of the 5e modules are the reality. The cult of Vena would be a very open and present force also. Basically Vecna won already and the PCs are the only ones who remember the real Faerûn. They will try to discover what happened, leading them to the wizards three and being sent into the past with a wish spell. There the book played out much the same from chapter 2 minus Kass. I watched a video from Game Masters where he suggests having a shattered obelisk that will teleport the party to Vecna be the main fetch quest and not the rod. I like this idea.

Does this sound like a good plan and is there more I should change? How do I have Vecna be more present throughout the campaign? Any advice will be greatly appreciated. Thank you!