Lots of old awesome series about Marvel Comics characters do actually exist.
Seems meaningless to make a post for each suggestion so I’ll group the ones I know well/I watched here.
Maybe you’ve seen them already, maybe not. I can’t know that, so here are the ones I’m willing to suggest you right now to pick from:
-X-Men: The Animated Series (1992)
-Fantastic Four: The Animated Series (1994)
-Iron Man: The Animated Series (1994)
-Spider-Man: The Animated Series (1994)
-The Incredible Hulk: The Animated Series (1996)
-Spider-Man Unlimited (1999)
-X-Men: Evolution (2000)
-Fantastic Four: World’s Greatest Heroes (2006)
-The Spectacular Spider-Man (2008)
-Wolverine and the X-Men (2009)
-Iron Man: Armored Adventures (2009)
-The Avengers: Earth’s Mightiest Heroes (2010)
Plus… MCU’s “What If…?” , since you reacted to WandaVision, Falcon&Winter Soldier and Loki? 😀