Honestly? I think it's possible that Usopp's speech IS Oda projecting a bit. But, not about Gear 5 Luffy.
...I think Usopp's speech could be about Oda, himself.
"Turning white or being crowned a Emperor hasn't changed a thing!" The "turning white" could be referring to Oda's hair turning grey as he ages. And the "crowned a Emperor" comment could be about One Piece unofficially becoming the "King" of Shonen Jump. (If only because all the other big-name manga already have or are close to finishing up.)
"He throws his life into every single fight!" This could be about Oda spending all his time and energy making every chapter of the manga. Week after week.
"He should have the freedom to die honorably on his own terms!" This could be about Oda being sick and tired of everyone being worried about his health? His fans. Even his editors and other manga authors, probably. Everyone trying to get him to finish the manga early so that he can finally take a rest. This could be about Oda wanting to do his story his way, even if he ends up dying before he finishes.
Even the line about “Nika this, Joyboy that, you’re even projecting Roger on him now!” Could actually be a subtle reference to other mangaka with health problems that we’ve learned about recently. Like Miura from Berserk, Togashi from Hunter x Hunter, and Toriyama from Dragon Ball. Oda is saying you shouldn’t be worried about HIM, just because people in his career have either recently died or developed severe health problems. He isn’t the same as them. His health problems may or may not be as severe as theirs were.
If you look at it in a metatextual way, Oda might be saying that people need to stop trying to protect him and just let him tell his story. No matter how long it takes. Just like how Scopper needs to stop trying to protect Luffy as a "chosen one", and just let him fight the way he wants to fight. Even if it leads to his death, it's okay, because he'll die doing what he loves.
...I'm PROBABLY overthinking this? But, I can't help but see some...parallels with how Oda treats his health and how Luffy treats his own health in the story. They're both kind of "reckless", in their own ways. And, after all these years writing the character, I could see Oda seeing a lot of himself in Luffy. But, again...maybe I'm seeing connections that just aren't there.