Grealish is better for Pep's system. His control, passing, IQ and more than anything the ability to push the ball closer to the dangerous areas makes him very effective regardles of what clueless G+A merchants say. I know hes a polarizing figure among City fans but I don't care, I say what I see. Same with Doku. You don't understand what kind of weapon you have in him. His ceiling is higher than Rodrygo. Rodrygo is a better finisher than Doku as of now, but hes not going to create nearly as many chances because hes not as athletic or great at close control when running with the ball. Savinho is the weakest of the bunch.
In the end it comes down to systems, roles and playing time. You swap Rodrygo and Doku between their respective teams for the previous season, both Madrid and City get better. Rodrygo had a very poor season for Madrid, but due to the tactical mess that Ancelotti accepted as a cost of keeping his stars happy, he was deployed everywhere - in the middle when Mbappe was out, on the left when Vini was out, but mostly on the right. He had a couple of weeks spell on the left in the middle of the season where he seemed to click with Mbappe, but he faded quickly, partly due to his own mental fragility and partly due to systemic tactical and personnel issues. He never had any consistency in terms of role or even presence/minutes on the pitch.
Rodrygo definitely reached his ceiling at Madrid, but I suspect he might find new motivation, better tactical guidance, more important role at City and all of this means it could work out. His forte is versatility. Although the left is his natural position since his younger days, Vinicius is better there, and Rodrygo doesn't get that much worse going from left to right due to his versatility and very automated and efficient style. But it its exactly because of this why Pep wants him I presume. He is coachable, has a good head generally and could benefit a lot from a more defined role and teammates who are also positionally sound.
Hard to say what Pep can do to make him better, but a more defined + important role and installed confidence could be a game changer. Hes a high IQ player, but ironically thats sometimes his biggest weakness - he is not daring enough, makes the most logical choice and doesn't try risks. For a player in his position this is needed a lot more. But I suspect he is aware of his physical shortcomings so thats why he made that his style. Hes more fluid than explosive.