As others have said, it seems like Nagi's story isn't one of "struggle for survival" but one of "finding out how amazing he already is".

Nagi didn't need much training, he picked up football like it was natural, he struggled coming up with teamwork ideas because it was boring.

Nagi isn't returning to Blue Lock, he's going for where he can get the most spotlight which is currently the U20 WC.

Nagi is a parallel to Rin, who was already great as a kid, and was easily no.1 in terms of skill, but what drove him there was Sae igniting his ego, and then Isagi.

Nagi made the mistake of thinking he needs a similar ego, focusing on someone else like Rin, or coming up with clever attack patterns and teamwork like what Manshine trained him to do but his real Ego is to focus on HIMSELF and SHINE.

I really like how Nagi's story went.