IMO the Fantastic 4 comic is bad, and Doom always has been a rubbish villain with a ridiculous costume. So bad movie = faithful adaptation.
This is very blunt but also I don't think you're really wrong, fundamentally.
The issue with the FF, which you simplify to "bad" is that it's outdated and irrelevant to almost everyone, and the characters aren't very relatable nor are they cool. Even where they approach relatability (Ben Grimm/The "rock hard" Thing - their words not mine lol), the same sort of thing has been done better by a huge number of characters. Ben Grimm is pretty low on the totem pole of "Oh woe is me, I got turned into a scary thing" characters. Mr Fantastic is big in Marvel, but how many smug, superior white men super-scientists have we seen as heroes or villains? I think a lot of his dialogue, if unattributed, would be very hard to distinguish from Tony Stark or even Dr Strange or even certain villains. Plus in the comic he's kind of awful. Human Torch is just yet another young male show-off with some issues about recklessness, and a particularly boring power set. The Invisible Woman has interesting but bizarre powers (invisibility AND force-fields - you can link them by making the invisibility being bending light with forcefields of course), but doesn't have a terribly clear personality because Lee wrote her essentially as one of the first "girl power" characters - literally he meant for the others to gradually realize that she was actually the most powerful of them all. Which was a bold move in the 1960s, but, like par for the course in the 2020s (Captain Marvel being probably the straight-up most powerful hero in Marvel, so long as we keep the godawful Sentinel off the table - Hulk and Thor are certainly somewhat comparable, but you get the point).
I swear to god the whole conception of the team was basically Stan Lee pranking people by making 1960s cultural stereotypes into actual characters, but that's a whole other discussion.
Dr Doom is a 1960/1970s icon, and whilst he looks ridiculous, he oozes 1960s style and has a wild vibe and completely insane powers (he's a wizard, seriously). But whilst many nerds adore this, I think most audiences, seeing an "faithful" Dr Doom played straight are going to be pretty "LOL???!?". Hence the awful versions of Dr Doom that we got in both the recent movies, where they where they managed to just create Dooms guaranteed to annoy anyone who liked the actual Doom, but also not at all compelling, just generic supervillains.
So yeah.
I did see one suggestion on the internet which could work, which is to make the movie a 1960s period piece for like, 30-75% of it (dangerously treading on Incredibles territory of course so I'd suggest a less stylized take on the 1960s), and then have Doom or Reed do something which pulls them all into the present day, irrevocably (with magic or science respectively), then have it become a sort of "fish out of water" quasi-comedy as everyone, including Doom, has to adjust to the new reality and find ways to fit in, and probably get into unfortunate situations with existing heroes/villains. This also has the benefit of allowing you to start largely in media res, rather than doing an origin story - you can flash back to a brief origin later if it's really needed - but I feel like people are willing to take superheroes "as they come" these days, and origin stories are only needed if they're vital to the personality and approach of the character - which definitely isn't the case with the FF. I think that's part of why the other movies struggled - the origin just doesn't matter to them in the shorter-term - it's not like Hulk or even Captain America where it actually matters, but they focused on that (especially the terrible Josh Trank one).
