1) After the events of the '84 Manhattan Crossrip, Peck launched a one man smear campaign against The Ghostbusters in order to turn New York against them with saying things like they (the Ghostbusters) are con artists and they used a bunch of helicopters to throw down 4 tons of marshmallow goo onto the streets. This eventually would lead to the lawsuits and suing of the Ghostbusters forcing them to go out of business in which Spengler mentions there was a parade followed by tons of litigations and lawsuits in the book.
2) After the Vigo incident, ghost sightings and calls died down with Janine eventually leaving to go do other work, Ray still working with his Occult book store, Venkman went back to do "World of the Psychic" until it was cancelled after five seasons due to declining viewership and Zeddemore opening his own firm as well as Louis Tully opening his own accounting firm "Tully & Tully brothers". This is where it gets extremely interesting as it foregoes the entire timeline of the video game (which y'all have said it did), and that by the end of 1991, around 1992 (not sure when since it's not documented, so guessing mortgage and eventually the firehouse does get sold to a coffee company who as compensation give the team one year of coffee (but they go the ghostbusters route of "three cups per week") and after Spengler and Stantz have an argument about Gozer coming back, Spengler has had enough, packs up everything inside the Ecto-1 as he can carry, steals the petty cash that's left but leaves all of the flightsuits that belong to Stantz, Venkman and Zeddemore as well as their packs and heads out to Summerville.
3) Spengler spends several months setting up the house as a fort whilst going up to the mining location and finds out that Shandor built in a failsafe in case anything happened in 1984 so Gozer could come back with Shandor inside the glass casket. This is around the time that the residents of Summerville start calling Spengler the "Dirt Farmer" which he embraces as he employs a "creepy old cook who believes in conspiracy stuff" gimmick, whilst hiding his stuff from the firehouse inside the location we end up seeing him direct Phoebe to as well as hiding the Ecto-1 in the garage. Spengler also notes that he was planning to make more inventions for the pack itself like an interchangeable wand that used the pink mood slime but add it to the pack itself (Sorta in a way referencing TVG's slime addon) but never got to start it due to Zuul killing him setting the events of Afterlife off.