I was an avid marvel fan as a youngling, collecting a number of books starting in the 70s. Fantastic 4 was never part of my collection unless a specific arc pushed my favs and that group together.
That doesn’t mean I was unawares, as it was very necessary to digest older Fantastic 4 yarns to truly understand how/why certain books/characters did what they did.
But it never interested me to buy or buy into the F4. As you said, I’ve never stumbled upon a die-hard F4 fan back in the day, whereas X-Men, Spidey, Avengers… had huge fan base.
I’m tangentially a F4 fan if only because without them we wouldn’t have Doom. If I had my way, Doom would be getting a solo film with the 4 only having cameos. I wouldn’t even want them as the primary antagonists, as I feel the real story to be telling with Doom is his feud with Mephisto for his mother’s soul.
I am a big fan of parodies or plays on them - Venture Brothers, The First Family, etc. And while I appreciate the Fantastic Four, when I go outside dressed as a stretchy man it’s always going to be Plastic Man, not Reed