That is exactly why the debate on values and principles is so important.
Even if citizens form constituent assemblies, we will do whatever we want if, during the debate, the first things discussed are the mechanisms and voting systems and whether we want one system or another. If we do not first agree on the values that underlie the system, we will have the dynamic that you described. We must produce a public declaration of the principles we want to uphold. Then, we can assess the specific proposals on the basis of these values rather than on one another's partisan interests.