Welcome to the community. There isn't enough context for specific advice, so I'll comment/question as best I can.
Are you interested in explanation or prediction?
You say you are seeking "optimization". This typically means you have already investigated the extremities of the design space and are looking for where in that design space do you get the best results. How have you already studied the design space (e.g., other controllable factors such as pressure, heating method, rate of temp. change, etc.) and noise (e.g., factors you are not willing to control such as ambient conditions, lot-to-lot variation of chemicals and purity, measurement uncertainty, cleanliness, etc.)?
What are the response variable(s)? Is it efficiency of synthesis? Or are there specific characteristics of the product of the synthesis you measure? In any case, have you studied the measurement systems?
I don't understand the following statements:
"my mixture don't like the idea of all adding up to a sum". Is your mixture a formulation? Are there other components to the formulation? In mixture designs, factors cannot be manipulated independently of each other. A change in the proportion of one factor will force a change to the proportion of others in the formulation. See here:
https://www.jmp.com/support/help/en/17.2/?os=mac&source=application#page/jmp/mixture-designs.shtml%2...
"I expect I will also need a hand trying to make these factors 'confounding'." Indeed if these factors are not able to be manipulated independently, you will need to take this into account. That is what mixture designs are all about.
"All models are wrong, some are useful" G.E.P. Box