In order to enter post-prime, you level needs to dip to a great extent.
Not really. Federer was already entering post-prime in 2012 when he won Wimbledon, as was Nadal in 2014 when he won RG. This is why both of them had poor follow-up seasons. Likewise, Djokovic's level dropped significantly during the latter half of 2016 compared to the previous year. All three are able to buoy their level back up temporarily (the duration of which depends on age and playstyle), but will never again dominate for entire seasons as they did during their prime years. Djokovic prime was 2011-2016.