do you see him anywhere?
what's he doing?
that's should answer your question on his relevance
The last time we saw Weevil, he was going after Marco. Why're we acting like something needs to happen now on the panel for us to assume it will happen? Again, I already said Marco is more than relevant enough even without any future involvements. Marco will continue to be Luffy's ally for life. And if there comes a time when Luffy will need all of his allies to bang behind him, Marco will be the foremost of them to show up. It's been already established at Wano.
he was a secondary antagonist in an arc who was experimented on 20 yrs ago . lunarian lore/exploits will be explored more. king's will most likely not because he isn't the focal point . he just happens to be part of a group that is
big difference.
He was a secondary antagonist in an arc that involves major players of the world. So that's as relevant as being the main antagonist in a separate arc and a less relevant arc if not better. And it doesn't matter when exactly the experiments happened on King, what matters is how it is relevant to the story now. The experiments on Kaido resulted in Momo while the experiments on King resulted in Seraphim, which in turn, resulted in shifting the world balance in the WG's favor. That's way bigger than anything Katakuri accomplished as a major antagonist with awakening and CoC. Again, King is directly involved in all of this. You can't separate him from it just because it's a racial ability. King is also the most relevant Lunarian, so saying King isn't the focal point isn't logical either.