Website Speed Optimization for Custom-Coded WordPress Site

I need an expert to optimize the speed of my custom-coded, heavy WordPress website (theme-based but with extensive custom CSS/JS). Currently, the Google Page Speed Insights and GTmetrix scores are very low on both mobile and desktop.

Goal:
Achieve 90–95+ scores on Google Page Speed Insights & GTmetrix without breaking its:
Existing functionality
Layout/structure
Styling/animations
Images quality
SEO setup & rankings
URL structure

Requirements:
Must have strong coding experience with custom WordPress sites
Must perform manual optimization (CSS/JS, critical rendering path, server tweaks, etc.)
No plugin-only optimizers - coding skills required
Only experienced specialists should apply

NOTE: Should apply only if you have handled complex custom WordPress speed optimization before.