The man with the title of pirate king will be the strongest character and was the strongest of his era. So if zoro is yonko level and luffy pirate king level after wano then there is no challenge left for them. Doesn't mean they can't take damages but still they are already too strong for their next opponents.
Do you really think shiryu is yonko level? And blackbeard isn't at pirate king level yet either.
You dont invent or reach new power lvl when you become PK, it is a navigation title showing that one reached Raftel.
In Roger's case it represented the one who explored all seas, Luffy wont do that, he didnt explore a lot of it.
Whether Luffy will be the strongest or not depends whether Oda wants to make Zoro and/or final villain and even Kaido stronger than EoS Luffy. Roger wasnt strongest of his era with Whitebeard around, at best his equal.
There is challenge left for them, any Admiral or Yonko, Mihawk is still a challenge, individuals which can kill them with a single hit. 2vs1 situation and they both are screwed. War setting like Marineford where cheap shots happen doesnt care how strong one is.
Blackbeard will never surpass Kaido imo and Shiryu will be/is around Admiral level.
I dont know what you even mean with PK level when such thing doesnt exist and represents something else...