I've been thinking about this for a while actually. It's quite difficult to make dinosaurs special in D&D without turning them into other monsters. Give them super powers, and they're not really dinosaurs anymore, yeah?
That said, I think this could work: first, for the brachiosaurus specifically, bump that STR and CON up to 30. HP should be in the 200s bare minimum, and give them multiple stomp attacks, a tail swing, and a neck swing that they can swap the stomps out for. That'll give them way more versatility.
Secondly, wrt all large dinos, give them resistance to ranged attacks, to represent their thick hide and blubber. This forces the players to engage in melee, where their terrifying power and strength becomes evident.
Then, give them lots of knockback effects. Shockingly few large monsters have the ability to throw enemies around with their attacks, so that mechanically sets them apart, and gives them a way to control the battle field that doesn't just turn things into a slug fest.
One of my favorite insights is this: consider that, in a population with a normally distributed size range, 50% the population is within 1 standard deviation of the average. This means that 75% is close to the average or smaller.
Consider then, that fossilisation is an incredibly rare process. All the conditions have to be just right, and the chances that the bones won't be destroyed in the intervening time until some paleontologist evolves and digs them up is slim.
Thus, statistically sleaking, most dinosaurs that we know of are almost certainly close to average.
Meaning that there's a strong likelihood that they could get even BIGGER