Comprehensive Business Management System Development

I need a customized business management platform that integrates multiple functions to streamline our operations. I run a non-profit and using multiple platforms everyday is annoying. Key functionalities include: - Managing customers, leads, and sales - Overseeing volunteers, their tasks, and daily operations - Handling invoices, expenses, and complete inventory system - Tracking meeting minutes - Voting on assistance applications that are filled out online - Accounting and inventory management User Access: - Managers: Full control over all aspects - Staff: Limited to managing their own tasks and records - Customers: Access to view their information and requests Ideal skills and experience: - Proven experience in developing complex management systems - Strong understanding of business operations - Expertise in user access control and security - Ability to create intuitive user interfaces for different user levels There are other items on the wishlist we can discuss. we can discuss further. I am looking for something very specific. Please provide samples of similar work done.