Full-Stack Website Audit & Fixes

I need a sharp full-stack eye on an existing Next.js / React site styled with Tailwind CSS and deployed to Vercel. The codebase has grown quickly and I want to be sure the foundations are solid before we add new features... (Budget: €30 - €250 EUR, Jobs: API Development, Full Stack Development, Next.js, React.js, Tailwind CSS, Vercel, Web Development, Website Audit)