Robust SaaS Retail Management System Development

I am looking for a developer/team to build a complete business management SaaS for a retail shop. The system should manage the entire business from one dashboard, including: 1. INVENTORY / STOCK - Add, edit and delete products - Product categories and subcategories - SKU/product code - Purchase price and selling price - Stock quantity - Low-stock alerts - Stock-in and stock-out tracking - Supplier management - Purchase history - Stock adjustment and damaged/lost stock tracking 2. SALES / BILLING - Create invoices/bills - GST billing support - Barcode/QR scanning - Discounts - Returns/refunds - Customer information - Payment methods: cash, UPI, card, credit - Printable/shareable invoices - Daily/monthly/yearly sales reports 3. PROFIT & LOSS - Automatic calculation of sales, purchase cost, gross profit and expenses - Net profit/loss reports - Product-wise profit - Daily, weekly, monthly and yearly reports - Expense management - Cash-flow tracking 4. TRANSACTIONS / ACCOUNTS - All incoming and outgoing transactions - Customer credit/udhaar management - Supplier payments - Receivables and payables - Payment history - Cash/UPI/bank transaction records - Automatic transaction entries from sales and purchases 5. CUSTOMERS - Customer database - Purchase history - Outstanding/credit amount - Payment reminders - Customer-wise reports 6. SUPPLIERS - Supplier database - Purchase history - Outstanding payment tracking - Supplier-wise reports 7. EMPLOYEE / STAFF MANAGEMENT - Employee profiles - Attendance - Salary management - Roles and permissions - Admin/manager/staff access levels 8. DASHBOARD The main dashboard should show: - Today's sales - Today's profit - Total stock value - Low-stock products - Expenses - Outstanding customer payments - Supplier dues - Cash/UPI/bank balance - Sales and profit graphs 9. REPORTS Reports should be exportable to PDF/Excel/CSV: - Sales - Purchases - Stock - Profit & Loss - Expenses - Customers - Suppliers - Transactions - GST-related reports 10. TECHNICAL REQUIREMENTS - Modern, clean and easy-to-use UI - Mobile + desktop responsive - Secure login - Role-based access - Cloud database - Automatic backup - Search and filters - Fast performance - API-ready architecture - Scalable so more shops/users can be added later 11. FUTURE SAAS FEATURES I eventually want to turn this into a multi-business SaaS where different shop owners can create their own accounts and manage their businesses separately. Please provide: - Your previous SaaS/ERP/POS projects - Recommended technology stack - Estimated development time - Development cost - Hosting/server requirements and recurring costs - Source-code ownership - Post-launch maintenance/support - Whether you can build the complete system rather than only individual modules. I am looking for a long-term developer/team, not just a basic demo or template.