Oda wants to finish his story. There's no time to give everyone the spotlight.
This point is key and unfortunately is the reason this happens. Yes, 150% Oda shat the bed with not just Post-TS, but pre-TS introducing a thousand characters in this story. Realize Egghead is comprised of literally ALL pre-TS characters save for Vegapunk (who still was referenced many times in Pre), and that should tell you where this problem started. He is dealing with the fallout of decisions he made over a decade ago with his world building.
And now, unless we want the series to last another 20 years, he quite literally has to addess all of the secondary characters he introduced and their plotlines. My assumption is Oda has key points where he intends the crew to still have defining moments in the story (i.e. Usopp probably getting Elbaf focus since its likely tied to his dream), but he sacrifices their presence in areas where they just kind of exist on x island.
Its a problem with making an adventure manga with 10 main characters and now he needs to resolve all major storylines in one final saga. All we can hope for is that if Akainu, Blackbeard and Shanks get focus, the crew also does too at those points. I dont doubt Imu and whatever final fights occur will give them all signficant focus and endings but even then we are expecting major wars with each of these characters lol
Writing endings to stories is hard. Writing endings to 30 year stories with no loose ends is nigh impossible. We are lucky he intends to end it, even if it means sacrificing screentime for other crewmembers.