I watched most of his games at Juve, especially in his first season because Juve were still serie a top dogs back then. He wasn't as good as in Madrid. Ronaldo was still an excellent goalscorer but also an aging player becoming worse and more limited every year. Benzema was offering so much more than him on the pitch at that point. Pretending that Ronaldo was better than Benzema in 20/21 is absurd; the frenchman finished 4th in the Ballon D'or that season - ahead of Ronaldo - despite not winning a single trophy and not being half the star the portuguese is.
There's no guarantee that Madrid would have fared any better with Ronaldo after 2018. Remember they had also underperformed in the league the previous season when he was still there. They were simply at the end of a cycle in 2018. It's hard to stay hungry after winning so many trophies, and playing 60-70 games per season for years, many of them high intensity european games, eventually takes a toll on players physically and mentally.
Now if we go into the details, Madrid did miss Ronaldo's goals right after he left, but many other factors can explain their dip in form: Zidane, the man who led them to 3 CLs in a row, had also left and they struggled to replace him, Hazard, who was supposed to replace Ronaldo, flopped, Bale became a part-time football player, Marcelo got old overnight and became a defensive liability, etc. The second season without Ronaldo, they won the league again.