And if players accumulate yellow cards over a series of games, they are suspended for a game, so there are indeed immediate consequences to getting a yellow card.
If there was more biting ears off and other body parts, that would make it more interesting. It would give the players a reason to go to the ground writhing in pain. Instead of a fake injury, they’d have a real injury.
Of course you’re correct that 5-on-4 is significantly greater an advantage than 11-on-10, but how much more significant is tough to say given the differences in the nature and the speed of the two games. But in any case, since soccer at the World Cup level has such closely matched teams and typically such low scores, the effect of any tangible advantage would worth exploring.
And the time period of a penalty box stay could be tweaked to give whatever results FIFA or some other governing body seeks - if the man-short team can play keep-away for two minutes, make the minor penalty four or five.
With regard to your other point, it all depends on what one considers “immediate”. In principle, you could be down to the last two minutes of the final game of the Cup, say up a goal, and commit a minor foul on your opponent with impunity - so you get a yellow card, so what? To a lesser extent, any player could have the same attitude in any game - you get one free foul, at least if it’s at the yellow card level, and if you’re worried about another one, you just back off a little, or your coach replaces you with someone at a comparable level, or whatever. I just think if a person commits an actual foul, there should be consequences then and there, not (maybe, or maybe not) later on or a game down the road