His best position is Busquets', period. He can drop between the CBS and help advance the ball. Distributing left and right with the odd cameo in the oppo's box. That's it.
At best you can use him as a b2b against lower level teams who can't put pressure on our backline.
He is poor positionally, both in skill and in style.
He looked good as something like a CDM in Ajax in an underdog team, most of it was him just being calm on the ball and dribbling through bullrushing opponents.
He has been here a long time now, he isn't good enough for a holding role. He can run around as a b2b and do stuff well on the fly, but you can't really have too many of those players at the same time, then you will lose against competent opposition. Pedri is a bit of the same right now, jogging around a bit aimlessly.
He can become Great as a CB, or very good in a free B2B role in a good team. I guess a double pivote suits him decently, but I don't like that setup at all.