The training system by me is fine and this type of games take you years in real life to become someone, that is just how it is and has always been, if you want faster results, play NBA 2K or some other this type of games (you can simulate games, just like in buzzerbeater and trade players etc. but much faster).
But there is one point that is good in your text, which is that the lower division leagues should be filled with active players and bots can fight against each other, would make game much more interesting from the very beginning. For example fill up one league and then start replacing bots in another one and so on. Because this is really annoying I have to admit to play whole season against only bots when there are tons of leagues also with only one active team.