CP Masako has the toughest transition. Adjustment disorder since 2002
Empress Michiko also had very difficult transitions. She reportedly suffered nervous breakdowns and lost her voice for several months in the 1960s and again in 1993.
Princess Sofia probably had the smoothest/easiest transition. She already represented a charity (as co-founder of Project Playground) and also had experience dealing with the media/public as a model and reality TV starlet. Becoming a princess expands her influence.
HGD Stephanie's slow solo debut is difficult for me to judge too harshly. She went from unknown to royal fiancée quickly and then, her mother died shortly before the wedding. Grief likely complicated the transition. Also, Luxembourg's royals have fewer events/patronages anyway and the Cour doesn't update their activities consistently. Luxarazzi reported Stephanie's first solo engagement as a jewellery store visit during the Japan economic trip in October 2014. In contrast, the Cour appears to have her first solo engagement in March 2015 since they did not mention her solo Japan event or her solo events during Germany's state visit in Nov 2014.
Princess Charlene had a hard time. She married the sovereign (not heir or younger son or heir's heir) and following Princess Grace and Princess Caroline as Monaco's "first lady" must have been quite a challenge. At least she attended events with Prince Albert much earlier.
Wasn't the Duchess of Cambridge's first solo engagement unexpected? She stepped in when Prince Charles had to attend a funeral. Not sure when her first "planned" solo should have been. Anyway, Catherine did well. The BRF and especially Prince William are careful about royal transitions given Diana's and Sarah's troubles.
I've read CP Mary's and Queen Mathilde's first solo engagement were quick. Mary after 1 month and Mathilde after 3 months of marriage. Queen Letizia unexpectedly had a solo engagement around 6 months (?) after marriage and then a long gap before the next one. Queen Maxima's first solo engagement was around 1.5 years after marriage and CP Mette-Marit took the longest at 3 years.