Name me one flashback that was segregated into 10 chapters with other plotlines in between
It's not been split up, you just haven't seen the start of it. Almost every major flashback has about ~1 page's worth of a look at it before it starts, with the character or other characters revealing details about it throughout the whole arc before it starts.
Take Law's flashback. Various characters hint at it, Law mentions more and more details e.g. him being a former member of the Donquixote family, Corazon existing, the white town etc. etc., then we get about a page of him as a kid with the grenades strapped to him as he joins the family. It's not until a volume later that we get the actual flashback. Same goes for Doflamingo. Vergo says "you don't know his past" in Punk Hazard, we get the Celestial Dragon reveal at the start of the arc on Green Bit, Doflamingo spoils that he killed his father etc. and we get numerous flashes of it before we get the full story.
90% of complaints I see on this forum whether it's gags or powerscaling or this are about things that Oda has literally always done.