I probably will have the Watchmen creators howling at me but here goes. To prevent the darker age of comics in the 90's I would start with toning down the Watchmen.
I think the Watchmen creators would be the first to agree with you that it, in large part, inspired the worst of the Dark Age. Moore has complained repeatedly that the form of Watchmen was ripped off widely, without any of the commentary or thoughtful content.
There are ways to do this without destroying the "gritty" feel of things. Examples: Have Dr. Manhattan destroy the nuclear missiles after launch, Ozymandias then makes it clear the Watchmen will not allow the world to be destroyed by either side. He also offers the opportunity to world leaders to meet at his Antarctic base to discuss the grievances that have triggered the war. This is just one suggestion. I would like to hear others.
The above being said, at this point we still have a Watchmen which features, among other things, a rapist superhero that works for the government and who murdered a woman carrying his child. And a psychopathic hero who chains a guy inside a burning building with a hacksaw, after said hero finds the guy fed a child to dogs. Watchmen is a dark comic. That's the point. Anyway, your changes would basically moot the story - without Ozymandias's false flag alien attack, and the Comedian witnessing its preparation, there's no murder of the Comedian, and no story. No Watchmen.
How about this: Dick Giordano takes Dennis O'Neil with him when he leaves DC in the early 70s, which aborts O'Neil's dark Batman run. Later in the 70s, DC hires the up and comer Mike Gold as an editor. He likes the job, but finds himself repeatedly prevented from going in the direction he wants. He works at Marvel for awhile, but that doesn't work out either, and he ultimately turns fulltime to his laudable political efforts and essay writing. No Mike, no First Comics. No American Flagg!, Grimjack, Nexus, Badger, or Dreadstar. After the DC Implosion, Jenette Kahn is fired, and replaced with a corporate suit. One who refuses to offer royalties to creators (no New Teen Titans, no Wolfman and Perez at DC, no Crisis on Infinite Earths). One who thinks American readers want to read American writers (no Swamp Thing, no Watchmen, no Hellblazer, no Black Orchid, no Sandman, no Animal Man, no Shade, the Changing Man, no Doom Patrol...). Creator owned comics rights are much weaker in this TL, so DC sees no reason to try and lure Frank Miller with Ronin (no Dark Knight Returns).
With less of a creator owned movement within the two major publishers, the dam finally breaks in the late 80s/early 90s with a huge exodus from Marvel to create alt-Image. This comes at exactly the wrong time, as Marvel's parent company is liquidating its assets. Marvel gets purchased at bargain prices by Disney, which launches several TV shows as part of the Disney Channel's transition from premium to cable. Marvel finds itself having to shape its comics to fit the tone of the Disney cartoons, and is further left foundering when it's caught in the Ovitz/Eisner feud. By the mid 1990s, comics are increasingly irrelevant, mired in campy nostalgia and subpar writing, as the new creator-owned studios struggle to find a market in an industry that has lost many of its would-be adult readers. By the turn of the century, the most well known comic related matter is EuroDisney's Spectacular Spider-Man's Swingin' Supercoaster, which killed five people in a 1998 malfunction.