Excel & Sheets Optimization Expert Needed

I have a large, formula-heavy file that lives in both Excel and Google Sheets. Over time it has become sluggish, occasionally throws #REF! and #VALUE! errors, and the user-facing buttons or links sometimes stop responding... (Budget: ₹600 - ₹1500 INR, Jobs: Data Analysis, Data Entry, Data Management, Data Processing, Data Visualization, Excel, Google Sheets, Visual Basic)