I don’t think he’s irrelevant to Oda, I think Oda is just being true to the characters. Sanji is an emotional character who has a lot of unresolved shit from his toxic family. While Zoro is a very stoic character, and his lineage would have little to no impact on him or his state of mind. I genuinely don’t think he’d really care.
The ice oni bullets wasn’t filler. It let chopper do something, informed us of Queens personality, and it dealt with Apoo. From what I’ve seen of the fandom on here it just loves to bitch and moan, so they way it looks at things has no relevance to me.
With the exception of whole cake island, Zoro has been critical to the success of the Straw Hats. With the exception of Luffy, I don’t think any other crew member has been consistently critical to their victories. They all play a part yeah, but not all those parts were required for their success. That wouldn’t be true if Zoro was irrelevant.