The fees would have to be paid out of revenue, so your model would be like:
User at start/ noob / level 0:
10% Fee on Interest
User at standard / noob / level 5:
9.95% Fee on Interest - 0.05% Rakeback
User at standard / noob / level 50:
9.5% Fee on Interest - 0.5% Rakeback
Or something similar. All of the project backers are already going to get half off some fees and with around 1400 of them that will make it difficult to scale that beyond some type of diminishing returns capped at something like 25% Fees Rakeback at high level.