WordPress Front-End Designer needed for a Mobile Optimization of a page

I need a seasoned WordPress front-end designer to breathe new life into (mainly) a single page that was built a over five years ago and never optimized for phones or tablets. The desktop version still works, but on smaller screens it forces users to pinch-zoom and scroll sideways - hurting readability and engagement.

Scope of work:
• Re-engineer the existing markup and CSS so the page flows in a clean single-column layout on mobile while remaining intact on desktop.
• Keep all core (text) content and images as they are; nothing should be lost, only rearranged.
• Refactor or replace any legacy code that blocks responsive behaviour - inline widths, fixed-pixel tables, outdated floats, etc.
• Ensure typography scales, line-lengths shorten appropriately, and white space is balanced for effortless reading.
• You are free to give suggestions of the usability and the look & feel of the whole website, as it has been created a while ago. I might be open for a bit more bigger renovation as a separate project.

Technical expectations
The site runs on the latest stable WordPress core with a classic PHP template (no builder). You are free to work in a child theme, custom CSS, or lightweight Flexbox/Grid edits - whichever delivers clean, maintainable code. No plugins, please (unless absolutely necessary). To ensure you have read the full description, please start your application with the word "SIBELIUS". I will only interview candidates who starts the application with the keyword.

Acceptance criteria
• Breakpoints display properly from 320 px up to large desktop.
• GDPR compliant.
• Google’s Mobile-Friendly Test returns a pass.
• Lighthouse mobile accessibility score ≥ 90.
• Page speed is equal to or better than before.

Please let me know your proposed approach and an estimated turnaround time so we can get started.