I don't think the "dip" in form is down to him being at Sassuolo too long, and it would look bad if his performances have dropped because of lack of motivation, given that Sassuolo have been fighting for European football this year. Berardi is probably going through normal inconsistencies you'd expect of a young player, similar to how Pogba was playing rather badly for a large part of this season.
Berardi would be the same age as Dybala if he joined this summer, which is not too late to make that move at all really. Berardi has the sort of Serie A experience that Giovinco didn't have until he was in his mid twenties either, and there is almost an entire career left in this player still. Should be interesting to see him introduced with a much higher standard of player than he's used to.