@wolfbrain
In the long term it is risky to focus on rewards. If rewards is the only reason players play and those rewards comes from players then it is really not sustainable unless for gambling type of fun where you accept to lose money for the chance of maybe winning alot.
With skill based rewards as you present it I think you will only attract a few players. In this case 1 of 10 will benefit and probably not that many will put in any extra effort to get the small reward increase. These are my initial thoughts and I might be totally wrong of course.
Another idea, also reward based, is to take the 0.5% of volume and raffle it to one or more of the players that have been active that whole week. Use the score to determine each players probabillity to win, higher score means higher probability. Then every player has a chance to win something significant. Even new players will have a chance, but their probabillity would be lower than someone with a higher score. It also incentivizes you to keep on getting a higher score.