I think it's more of a symptom of Oda focusing on way too many characters at once rather than him losing interest in them.
Dressrosa had some great moments for Usopp and Franky but completely sidelined Robin, WCI was great for Sanji, Brook and even Nami to an extent but Chopper felt like a side character. Wano has focused a lot on Zoro and Sanji and at least gave some of them 1v1 fights, but as you said Usopp and Brook are just there.
The sheer scale of the conflicts we've had since Marineford (almost every major arc since then has been a war of some kind) has taken away a lot of focus from the SHs as a group sadly.
Dressrosa had some great moments for Usopp and Franky but completely sidelined Robin, WCI was great for Sanji, Brook and even Nami to an extent but Chopper felt like a side character. Wano has focused a lot on Zoro and Sanji and at least gave some of them 1v1 fights, but as you said Usopp and Brook are just there.
The sheer scale of the conflicts we've had since Marineford (almost every major arc since then has been a war of some kind) has taken away a lot of focus from the SHs as a group sadly.