Website Functionality Enhancement Needed

I’m ready to move my current site beyond simple cosmetic tweaks and focus squarely on its underlying capabilities. The layout and content are staying as they are; what I need now is clean, reliable code that extends—or replaces—key features so the site actually works the way visitors expect... (Budget: $250 - $750 USD, Jobs: API Development, CSS, HTML, JavaScript, Node.js, PHP, Python, Web Development)