Credits, and rewards in general, need to be significantly improved for just Sport Mode only. For what is supposed to be the primary mode of play for the game, the rewards given are not enough for the average player to remain motivated enough to keep participating in and finishing races, as well as keep on playing Sport Mode races.
Say if a player starts in 14th and moves up to 10th; telling the player that they moved up four positions and that they had a great race because of it isn't really that captivating of a motivator for the average player. GT League is where the classic Gran Turismo gameplay is at. You're able to tune your cars within regulations and sweep the competition to reap the rewards if you want to; that's how the classic gameplay has always been for the series. But for something like Sport Mode that takes players out of that confined comfort zone and pits them against real-life competition where constant skill and attention are needed and the outcome is never guaranteed, the rewards for participating and finishing these Sport Mode races should feel like a more rewarding experience than just "You moved up a few spots! Good job!"
Perhaps they can add a small percentage multiplier on rewards for every position they moved up, for every clean race, and perhaps even for their clean race streak. Percentage multipliers for no penalties wouldn't be fair due to the sheer unpredictable nature of online racing. But in general, they need to increase rewards for Sport Mode races so that average players can keep coming back to the game mode to enjoy more races, hopefully improve their skills, and best of all, have fun doing it. Giving a race hound a small kibble of food after running several laps around a stadium full of other race hounds isn't going to make him that happy, I would think...