Is it permissible to annul the civil marriage while maintaining the Sharia marriage in order to be able to marry another woman, bearing in mind that the wife has embraced Islam and is infertile, and the country does not permit polygyny, and the wife might revert to her former religion if he marries another?

If you are able to marry a second wife without infringing upon the rights of the first wife or causing significant harm, then there is no impediment. You are not obliged to refrain from marrying a second wife due to an expectation that your first wife might apostatize. However, if that were to occur, it would indicate a weakness in her faith, and you should teach her matters of her religion and guide her towards faith and righteous deeds.