Less room to operate meaning less spaces? That's literally Grealish's strength. He has amazing close control and creativity. What he might struggle at City is with their possession based style of play, but I think he will adapt pretty fast.
As for your assertion that he's not among the very best in PL, that's a hard no for me. He is the most creative player in PL from open play, he is the most fouled player by a distance, and he literally carries Villa. Among attacking midfielders only Bruno and KdB are his peers. Stats back that up too, in the whoscored rating he's top 3 in the whole PL behind Kane and KdB(although whoscored is biased towards attackers).
And he has couple of great seasons because before that he was not in the PL. He's a late bloomer for sure, but he is just entering his prime, and his game doesnt rely on pace. If he can stay focussed(he does have off field issues tbh), he should continue this for years. He's year younger than Bruno.