WordPress/PHP troubleshooting - Fix 502 Bad Gateway and Upgrade Legacy Site

We need an experienced WordPress/PHP developer to diagnose and fix performance/outage issues on our corporate WordPress website, trianglecapitalllc.com, hosted on Nexcess. Our hosting provider has identified that the site is running on an old PHP version and has multiple outdated WordPress plugins and theme-related components. The site is currently slow and intermittently producing 502 Bad Gateway errors. We need someone who can safely stabilize the site, update the WordPress/PHP/plugin environment, and make sure the site remains functional after the updates. Important: This is not a redesign project. We are looking for a technical WordPress maintenance, troubleshooting, and upgrade specialist. Scope of work: Review current WordPress, PHP, theme, plugin, and hosting configuration. Diagnose the cause of the 502 Bad Gateway / slow-site behavior. Create or confirm a full backup before making changes. Set up or use a staging environment if available through Nexcess. Update WordPress core, plugins, and themes safely. Evaluate old or abandoned plugins and recommend replacements or removal where needed. Test compatibility with a modern PHP version before upgrading PHP. Upgrade PHP to a supported modern version if safe to do so. Check the site after updates, including page loading, contact forms, menus, images, sliders, SEO plugin, and admin dashboard. Review caching/CDN/server settings that may affect performance. Provide a written summary of what was fixed, what remains risky, and what ongoing maintenance is recommended. Current known plugins include: Contact Form 7, Classic Editor, Slider Revolution, WPBakery/Visual Composer, Ultimate VC Addons, Yoast SEO, WP File Manager, WP RSS Aggregator, Admin Menu Editor Pro, and various theme-related plugins. Required experience: Strong WordPress troubleshooting experience. PHP upgrade experience, especially moving older sites from PHP 7.x to PHP 8.x. Experience with Nexcess, Liquid Web, or similar managed WordPress hosting. Comfort working with legacy WordPress themes and older page builders such as WPBakery/Visual Composer. Ability to work safely without breaking the live site. Familiarity with staging sites, backups, WP-CLI, error logs, debug mode, caching, and plugin conflict testing. Please answer these questions in your reply: Have you handled a WordPress site with 502 errors before? What was the cause? Have you upgraded legacy WordPress sites from PHP 7.x to PHP 8.x? How would you approach this without breaking the live website? Are you comfortable working with Nexcess hosting? Can you start with a diagnostic review and give us a clear action plan before making major changes? We would like to begin with an initial diagnostic and stabilization phase, then proceed to updates and cleanup once we understand the risks.