I firmly believe Luffy is still a rubber boy and will continue to be one even though he is now confirmed as Joyboy. Luffy’s awakening will not mean that he is no longer a rubber boy.
I do not believe that Luffy fully died when Kaido smashed him into the ground. Luffy’s heart stopped and his body began into a near death state when Luffy remembered his fight back on Skypiea with Enel.
Enel had his heart stopped but was able to restart it before fully dying using his electricity. Luffy having rubber organs he has already proven to know how to manipulate (specifically Gear 2) was able to regain consciousness in time and restart his own heart like Enel did.
Skypiea is not only one of if not the earliest foreshadowing between Luffy and Nika but is also one of the few times Luffy defeats a villian in a way that doesn’t really seem to force him to grow at all. Luffy was always going to win because he was rubber and Enel couldn’t really inflict real harm with his usual techniques.
On Dressrosa Doflamingo helps expand upon the Awakening concept introduced during Impel Down, describing it as something that comes from “mastering” a devil fruit power entirely.
In Wano, we have already seen Luffy master the outer limits of his rubber abilities with the advanced Haki infusions he learned to be able to damage Kaido. I believe Luffy restarted his own rubber heart and this represents him achieving complete mastery over both his outer rubber abilities and his internal rubber abilities like control over his organs.
The “drums of liberation” that Zunesha hears is the sound of Luffy’s heart beat picking up to an extraordinary pace only a rubber body could withstand. Luffy now armed with complete mastery of his rubber body will now be able to ramp up his heart rate to the extreme, giving him access to more fire/heat abilities to solidify the Nika connection, more energy and explosiveness in his gear forms, and the new liquid like power we see around Luffy at the end of 1043. This new power is natural rubber that Luffy can now melt into a freeflowing liquid state much like Katakuri’s mochi.
Luffy isn’t the first Gum-Gum user in 800 years, just the first to be strong and tenacious enough to push it to its absolute limits like Joyboy did.
To me personally I think this is the most satisfying way to explain Luffys seeming resurrection, his previously useful rubber like qualities (because he’s always been/is natural rubber), and justify his new power up in a satisfying rubber based way.
Theory by GoodOlSticks (https://www.reddit.com/user/GoodOlSticks)