That's fair. I agree there's much potential for Brook this arc. And it's kind of the deciding point for his character for me.
As for Robin, not true. We didn't know the Poneglyphs are a map to Raftel, but they have been said to have knowledge on the weapons as far as Alabasta. Crocodile recruited her specifically for the purpose of reading the Poneglyphs which shows it's a rare and useful commodity. Robin has been useful for that alone since Alabasta. And especially since Enies Lobby.
Robin has knowledge on 2 ancient weapons and their whereabouts. Not once has it been important for the crew in any way shape or form. Oh it will be eventually but certainly not in the last 14 years since she's been in the crew.
Oh and we knew all along. We weren't told explicitly but we knew since the very ending of Skypiea. Roger mentioned that he would guide the documents all the way to the end or some such. Then there is the fact that speculation about getting to Raftel has been rampant since the meeting with Crocus. Everyone and their mama understood right away that people don't just sail to Raftel, there was some trick to it. These same people(darn near everyone) knew it had to do with the Poneglyphs guiding the way.
It's a useful skill absolutely but not a necessity for living aboard the pirate ship or even making it to Raftel. Even the Roger Pirates managed to get to Raftel and decipher all the Poneglyphs without the skill that Robin has. Luffy even inherited that skill. That's not to downplay Robin because on a meta level that stuff is extremely important for the reader in the mean time.
Brook is a musician. His job is playing music and providing entertainment to a crew that already has the most entertaining captain. I don't think it's narrow minded to think this. All of the crew members usefulness is becoming increasingly obvious, especially in this arc - Zoro's leadership, Robin's archaeology, Franky's building and craftsman skills, Usopp being the sniper and the only one capable of helping beat Sugar, the absence of Sanji resulted in Luffy almost dying of being poisoned because they don't have a dedicated cook anymore, Nami's navigation skills helped them reach Big Mom's territory, Chopper kept Luffy alive when he was on the brink of the death.
And then there's Brook.
When he first joined I thought he was great but now he feels like an odd man out. He almost feels forced and as if he has zero purpose.
I get what you are saying but that is the nature of his primary job, to entertain. He's still a fighter. He is a Straw Hat through and through and the guy Luffy has been requesting since the East Blue. Brook is a musician who entertains and with those same skills of entertainment he can fight. Usopp's primary role is that of a sniper, it's not like he's constantly at work 24/7. How is he any different from Brook?
Yeah Usopp was the only one in that moment to be able to defeat Sugar but how is that any different from Brook saving the others from Jiola/Jora? Because Brook played music and understands the artists soul he was able to trick her and defeat her. He protected the crew with his musical fighting abilities.
Entertainment is a powerful thing and Brook has literal power backing his version of it. But until Brook has to unlock One Eye'd Willy's secret cave by playing the proper notes on an ancient piano organ I guess anything up until then just doesn't count.
Zoro's leadership, the guy who would have gladly let Smoker drown and would rather leave Sanji behind.