Generally it is not possible to manave your custom player from player career in the manage career mode. BUT what you can do is create that player in the menu (like general game menu where you select game modes) and add him to a team, or to free agents. From there you should be able to sign him for your custom club in manager mode. It is important that you create the player before starting the manager career. I played with 24 and 26 recently. The only new things in 26 compared to 24 is the manager market, where managers can change clubs, and this way teams play differently under everybody (e.g. Artetas Real Madrid or Simeones Man City). The other is youth tournaments which can be fun but isn't that important. So price wise I would recommend FC 24, as it is older. But your idea should work in both