you can optimise the enzyme either by increase level of substrate with constant level of enzyme or by various level of enzyme with constant level of substrate.

In order to study the effect of increasing the enzyme concentration upon the reaction rate, the substrate must be present in an excess amount; i.e., the reaction must be independent of the substrate concentration. if the amount of the enzyme is kept constant and the substrate concentration is then gradually increased, the reaction velocity will increase until it reaches a maximum. The minimum level of enzyme which evokes maximum amount of products from substrate should be optimum level.

Yes, you can definitely decrease your enzyme concentration well below 0.5 uM; your enzyme may even be more stable in a dilute solution. In graduate school I frequently ran discontinuous assays with an enzyme concentration of 50 nM and excess substrate.

Besides reducing the concentration of enzyme, you could use a lower temperature.

Also I wanted to add, that if you use low amounts of enzyme, and it really depends on how sticky your enzyme is, that you may want to add BSA or a small amount of detergent. The amounts are empirically determined. But I would start with 0.1% BSA. For detergent, Brij-35 works. We have run assays with enzyme concentrations less than 0.1nM