If Osimhen plays in the Premier League, he will be recognised as one of the best strikers in the league because he can score goals from any position. However, going to Galatasaray is also a good option because they value him there and paid his release clause to get him. Galatasaray is playing in the Champions League this season, which is another opportunity for Osimhen to prove that he is one of the best strikers in the league right now.
A success story in a league doesn't mean it will be repeated in another league. Even in the same league, a player can be top with one and fails to shine with another, depending from the coach, his relation with his teammates, adaptation with the new conditions and strategies used.
English Premier League is very competitive and nothing is guarantee there even with top players. Haaland for example was expected to do way better than what he is doing right now.