Could sex alone drive complexity?
Replace the word "complexity" with any other word..."height", "weight", "resting metabolic rate", etc. and the model would still be solvable in a mathematical sense and it would read the same way. So, it's hard for me to accept the model as relevant to the evolution of complexity.

I think you need to make the model have a more robust definition of complexity (as mentioned in the comments) but you also need to consider the costs and benefits of complexity in your model, which will greatly affect the dynamics and actually carry biological relevance.