When a base set of libraries is installed on a cluster, can indeed conflict with a notebook submitted to the cluster that defines a conflicting set of libraries for installation. This is because the libraries installed at the cluster level take precedence over those specified in the notebook.
If you need to use custom Python libraries or modules, the recommended approach is to install the required libraries directly to the cluster using the cluster library installation feature. This ensures that the libraries are available to all notebooks running on the cluster.
About your pythonpath variable, please see;
https://kb.databricks.com/en_US/clusters/cannot-set-a-custom-pythonpath