OpenCart 3 Core Session, Cart Sync & Checkout Payment Method Bug Fix

We run an OpenCart store (hosted on a cPanel VPS) and urgently need a verified OpenCart specialist to fix a critical session/checkout breakdown and optimize our store images today.

The Context:
A full-page caching extension (Lightning) was uninstalled. Since its removal, several underlying OpenCart core issues surfaced that need immediate resolution.

Issues to Fix:
1. Session / Login Failure: The server is failing to properly write session files to the session directory / `oc_session` table. Customer login is broken or getting dropped.
2. Cart Sync Glitch: Adding/removing products via AJAX causes the cart count/dropdown to reset or revert to stale data.
3. Checkout Error: At checkout (Step 3), selecting a payment method throws "Warning: Payment method required!" because the selected method is not persisting to the session.
4. Performance & Cleanup: Clear all dead OCMOD hooks/modifications left behind by the uninstalled caching extension, verify PHP OPcache settings, and ensure the store runs smoothly without external cache bloat.
5. Image Optimization (WebP Conversion): Convert and serve product, banner, and store images in modern WebP format without breaking responsive layouts or theme styles, improving Google PageSpeed and overall load times.

Requirements:
- Deep expertise in OpenCart 3 core architecture, OCMOD modification engine, and OpenCart session handling.
- Experience with image conversion/compression tools and serving WebP in OpenCart.
- Strong Linux / cPanel / PHP configuration troubleshooting skills (permissions, session.save_path, slow query tracking).
- Must start immediately and be available to debug directly via cPanel / FTP / OpenCart Admin.

To filter out automated bot bids, please begin your proposal with the phrase "OPENCART EXPERT".

This is a fast turnaround job. If you can fix this within 12 hours, place your bid.