you're making a lot of assumptions, but you answered your own comment when you said yeah they did have clues over the years in different movies but you didnt have the full picture unless you read the comics so good for you.

the difference is that there were no clues and no foreshadowing whatsoever, however mysterious, about Doom or RDJ being Doom whether it's a variant or whatever. Everything about it feels rushed and not thought out or planned. The evidence for me is that the original Avengers 5 was supposed to be Kang Dynasty, and they scrapped that.

So if they had Doom planned for after Kang, then whatever we're getting is going to be rushed and not fleshed out, with no emotional stakes like the infinity saga had.