Unpopular opinion, but a lot of these guys who "became better players" after leaving our club, didn't actually get better. They just look better because those clubs are more successful. It's quite a common line of flawed thinking where individual success and team success get conflated.
Addo-Carr wasn't picked for origin last year because the Dogs were coming last, even though he wasn't any different as a player to the guy he was in Melbourne. Fittler isn't a great selector admittedly, but a lot of people put a line through him for that reasoning which was completely stupid.
I actually think Tedesco was at his absolute best in his last years with us. That guy carried our team and would routinely score some freakish tries. Obviously still an incredible player at the Roosters, but I don't think he actually does anything better now than he used to with us except he's probably more of a leader and there's been gradual improvements in intangible aspects (e.g. reading the game, positioning, composure) which come with age and experience rather than the club you're at.
Addo-Carr is another example - Melbourne didn't make him a superstar with their coaching, they just took an extremely physically gifted guy and gave him opportunities to succeed while living through the bad. There's probably certain things which Bellamy identifies better than other coaches, but I'm certain Addo-Carr would've become a great winger here too if afforded the opportunity. Instead Taylor the dope continued to play Jordan Rankin over him.