The formula is a little different but the role is the same, deal with it.
In DB and DBZ there is this special dragon who has the ability to make people's wishes come true, entire arcs (like Namek) are based on people going after his incredible powers. Is Shenron, or let's even say Junior, more important than Vegeta? Nope. It is panel time and quality panel time that determine how much important for the story a character is and both Robin and Sanji are below Zoro (even if of course Sanji is way closer). Robin is just one of those straw hats neglected by Oda, he could have give her focus for the poneglyphs in Wano but basically she get nothing, and almost every SH shares that fate.
More panel time with more quality.
Zoro has his WSS dream, I'm not gonna argue against Killua or HxH cause the quality there is above OP but what do Sasuke or Vegeta have above Zoro? They both are obsessed about the main character and exist in function of him, Sasuke had all his brother issues but Kishimoto tried a bit too much until the final clown fiesta where he wanted to become the hokage or start a revolution out of the blue and stuff like that while Vegeta drive is to surpass Goku, there isn't litterally anything else.
Like the Vegeta and Sasuke, he has the WSS which is his little personal plot and that is it. I'm not denying this manga is Luffy piece or that Zoro is something uber exceptional but he falls in the same exact category as other famous shounens 2nd guys.
This is more debatable, I give you that. In stories like DBZ or Naruto there are way less characters and also the main cast is smaller, in OP there are almost always the 10 shs as ""protagonists"" in every arc plus a plethora of secondary characters that have to be feed and also a main character who is eating more and more space. So Zoro doesn't get much in comparison to Luffy but his role is underlined in every arc, he is not a main character like Luffy but you can't even put him on the same pedestal as people like Brook, or Robin, who are barely noticeable in an arc (sadly). In the pre time skip it was more evident but it is still there (just look at how he shined in Wano fighting wise). Vegeta or Sasuke can seem way more relevant in the story but in the end for DBZ and Naruto the main characters are Goku and Naruto (maybe in Super Vegeta is getting more attention but I'm not able to read that stuff).