He has to solidify his decision from now because of course even if the hope of joining a big club in another domestic competition is also enough to make his name rise but the worst possibility of declining performance must be considered.
Some players have to be an example, like Pogba or maybe Lukaku, who had a little trouble and lost his fame when he was at a new club, so in this case he really had to find the best way for his future.
The two examples you gave are absolutely right, but not all the best players are like that. So I won't equate Osimhen with the two players you mentioned as examples, because there are also players who when they go to other big clubs become more famous with their good performances that never decrease or disappear. So don't just look at the bad examples, but try to see some good examples from other players so you can be a more accurate comparison for this.
Even though these are not the same, we must also be aware that the playing conditions for Serie A have a quite different tempo so when you are in a new club like La Liga or even the EPL which has a higher level of intensity, especially in that tempo, you must also think about it carefully.
Indeed it would be great if he could do well when he was playing at a new club but when he didn't this could also kill a career which was actually still classified as still being able to shine even more.