So, my speculation on timing/events based on what we've seen ...
(1) During the 5-year gap, Vision's body is reclaimed by the US and ends up in SWORD's hands. I'm surprised Wakanda would give up so much vibranium, but everything would've been chaos and without Shuri, no one could restart him.
(2) Everyone returns, Thanos defeated, Tony's funeral, Wanda goes looking for Vision.
(3) Wanda finds Vision, frees him from SWORD's diabolical plans.
Wanda would not be able to restart Vision. Someone (who isn't Agatha) shows Wanda how to live her dream life with Vision.
(4) Wanda and Vision end up in a house in Westview NJ.
Mystery villain pushes Wanda w/Vision towards Westview. Maybe it's a special place, a physical nexus? Hex and sitcoms start. If Nightmare, maybe Wanda dreams about that kind of idyllic life from seeing US reruns in Sokovia and he makes it real.
(5) Agatha arrives as sitcom reality starts, causes trouble, kills dog.
She could be an agent of the big bad but I'm going with this being multiple agendas. Agatha is drawn by the magical energy. She monitors Wanda, wants to use her for her own purposes, maybe wants to disrupt big bad's plans. Pietro was trying get intel from Wanda, so if he works for Agatha, then Agatha doesn't know how the hex started.
For mystery villain and Agatha, Wanda is the prize as a Nexus being (based on the ad).
For Hayward, Vision is the prize.
For Wanda, it's Vision + kids that are the prize.
No one except Darcy/Monica/Jimmy/Vision care about the people of Westview.
At some point, we get a moment where the people of Westview surround the heroes and defend them, like the New Yorkers did in Spider-man.