You can add customer_id(entity_id) in export CSV using following way :
You just simply add entity_id field in $validAttrCodes array in following file:
You can change in public function export() function.
public function export()
$collection = $this->_prepareEntityCollection(...
You should try another way. First of all, please upgrade your Magento version, you can upgrade Website in 2 instances one from Composer and Another from Manual Way(Downloaded form Magento Website).
If you choose a composer update, you can consult this document from Magento: https://devdocs.magento.com/guides/v2.3/cloud/project/project-upgrade.html
If you ...