I think with what he imposes on himself, he'd be better off just becoming a total writer and offloading the majority of the artwork on a main artist. Monthly publication would end us with between 30 pages and 40 pages, which would be the equivalent of two chapters a month. He does roughly three chapters a month. I don't think it's enough of a change.
And god, I bet he'd fill his new free time with something outrageous, like more supervision on any of One Piece's media arms.
Yeah, in my humble opinion, I think Oda's best shot at extending his life is to drop art duties and adopt full supevisor duties if he needs something to do.
The good think about supvesiory duties is that it mostly requires communication, which he likes doing already, given the annecdotes about how much hours he spends speaking with editors, voice actors, and shueisha staff.