I think the IM2-IM3-AoU-CA:CW era RDJ phoned it in big time. Mostly because the character stayed stagnant and RDJ didn't have a lot to do except for being a non-stop quip machine most of the time. If he took one step forward and then it was nullified by a backward step in that very movie or the next one. There wasn't much progression (not much of regression either tbh) for Stark Jr.
But Infinity War and Endgame humanized his character and provided a very good closure to his journey. It helped that Marcus & McFeely focused a great deal on Tony in these two movies but they made it count and RDJ brought his A game when it mattered the most. He earned his moment.