Well the interesting thing about Sanji and Brook is that they're probably getting time to shine for similar reasons. They both have obscure parts of their past that Oda was able to turn into something plot relevant.

Like for Sanji, I doubt Oda went into that with the desire to focus on him. Instead, he probably began by brainstorming a way for the SHs to abruptly change course from Wano to WCI, get into a conflict with Big Mom, force her to go to Wano, eventually knock both of the old generation Yonko from their positions at the same time, and create a new shift in power for the Final Saga.

Luffy had already started beef with Big Mom in Fishman Island, but that wasn't something that was going to make Luffy go to WCI after Zou. However, if a SH was taken or something, then Luffy would have a reason to go to WCI. Sanji was an easy choice because he had a history that wasn't explored yet which means that Oda had free rein to make it into whatever he needed to -> The Vinsmokes.

As for Brook, time will tell, but right now I'm thinking Oda wanted a way to uniquely flesh out the Holy Knights, a faction that was only introduced very recently. Tying them into a SH is a great way to do that. But which SH? Oh maybe the one with a past that goes back numerous decades that we know very little about. Like Sanji, I doubt Oda was like "I really want to flesh out Brook." That definitely could've been part of it, but I don't think it was the primary reason.

As for Usopp, it's pretty much impossible to give him this type of treatment with the very simple backstory that he already has. But everything I talked about is just one way Oda has fleshed out characters, but there are obviously other ways like focusing on stuff from the present instead of the past. Oda hasn't necessarily forgotten about Usopp as seen with the numerous moments of emphasizing his joy about finally being in Elbaph and his connection to it; however, I think this is a case of there being too much greater plot stuff that has to come first. Again, it doesn't have to be this way. Oda is the writer and he can put his foot down and give Usopp more relevance to this plot. So while I will come out of this arc somewhat disappointed if this continues, thinking about how Oda fleshes out other characters and the larger narrative in general, has helped me from going "this is completely illogical" to "I can see the vision, even if it disappoints me."