Resolve Server Errors Triggered by Email Campaigns

I am facing a critical issue with my WordPress website hosted on a dedicated Kinsta server. Every time I send email campaigns via ConvertKit, the server experiences a significant spike in errors (500, 502, 503 & 525), leading to downtime and disrupted user experiences. This issue is persistent despite multiple attempts to address it with Kinsta support, and I need expert help to resolve it.

BACKGROUND
-> Hosting: Dedicated Kinsta server (C2-standard-8) with 8 CPUs, 32GB RAM, and 20 PHP workers.
-> Emails: Weekly campaigns to ~40,000 subscribers through ConvertKit.

Error Details:
-> Errors predominantly occur immediately after an email campaign is sent.
-> High traffic spikes from email tracking links (with query strings like ck_subscriber_id and utm_ parameters) overwhelm the server.
-> Admin-ajax requests appear to be a contributing factor.
-> Disabling caching for ck_subscriber_id cookies may be causing conflicts when combined with UTM parameters.

Additional details:
-> Code snippet plugin previously caused errors (now resolved by deactivating it).
-> Ongoing PHP worker and cron job timeouts have been reported in the logs.
-> There is a large spike in cache MISS status when emails are sent.
-> Attached is a chat thread with Kinsta providing fuller details.
-> Here is an example of the URL string from our newsletter - https://www.conservation-careers.com/job/conservation-detection-nosework-seminar-2025/?utm_source=convertkit&utm_medium=email&utm_campaign=Unmissable%20opportunities%20in%20Conservation%20this%20week%20-%2015873629

OBJECTIVES
Root Cause Analysis:
-> Investigate the server logs, performance, caching and traffic behaviour during email campaigns.
-> Identify specific conflicts or configurations causing the errors.

Optimization & Fixes:
-> Resolve any issues identified in the Root Cause Analysis. This may include the following,
-> Addressing conflicts between caching rules, UTM parameters, and ck_subscriber_id.
-> Optimizing admin-ajax usage to reduce server load.
-> Implementing efficient caching rules for email campaign links.

Testing & Monitoring:
-> Monitor server performance during the next email campaign to confirm the issue is resolved.
-> Provide a detailed report of changes implemented.

Recommendations:
-> Suggest further performance improvements for WordPress on Kinsta hosting.
-> Propose preventative measures for scalability during high-traffic periods.

KEY DETAILS
Plugins involved: ConvertKit, Paid Memberships Pro - ConvertKit Integration.

Past Kinsta support actions:
-> Temporarily disabled caching rules for ck_subscriber_id.
-> Recommended enabling pre-caching for campaign URLs.
-> Restarted PHP and adjusted NGINX settings to handle traffic better.

Deliverables:
-> A detailed report of the issue and resolution steps.
-> Implementation and validation of fixes to prevent recurrence.
-> Recommendations for improving server performance and handling high traffic.

Requirements:
-> Proven experience with WordPress performance optimization, particularly on Kinsta hosting.
-> Familiarity with ConvertKit and handling query string conflicts in email tracking.
-> Strong analytical and troubleshooting skills, especially with PHP worker management, caching, and admin-ajax optimization.
-> Clear communication and ability to document solutions.

If you can help resolve this issue and enhance my server’s performance, please provide a brief outline of your approach and relevant experience.