You could try a sparse experimental design using Taguchi (1986, 1987) orthogonal matrices (see for example https://www.itl.nist.gov/div898/software/dataplot/designs.htm#taguchi and doi:10.1002/(SICI)1097-0134(199710)29:23.0.CO;2-N). This allows the test of k factors, each with n different values, to be done in 2 k + 1 instead of nk experiments.

Kingly check https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/Book%3A_The_Basics_of_GOB_Chemistry_(Ball_et_al.)/18%3A_Amino_Acids_Proteins_and_Enzymes/18.06%3A_Enzyme_Activity

Also check https://www.sciencedirect.com/science/article/pii/S2213020914000068

Also check https://www.sciencedirect.com/topics/engineering/enzyme-concentration

Thank you all for your answers.

For a quick and simple check, normally enzyme assays use substrates at between 5 to 10 times the Km value. If your enzyme is from a novel source, the concentration of substrate can be adjusted provided no substrate inhibition occurs. Doubling the concentration of enzyme also doubles the maximal activity unless you are reporting your results as specific enzyme activity (per mg enzyme). Increasing the concentration (amount) of enzyme can immediately increase the enzyme activity.

If you are familiar with statistical optimization (e.g. DesignExpert software) you can start with Plackett Burman to screen for significant factors (normally >5 factors) and then proceed for Box Behnken or CCD.