Bulk WhatsApp Marketing Automation Setup

I have a list of roughly 50 000 opted-in contacts who reached the final stage of my marketing funnel, and I want to push out a single campaign to all of them on WhatsApp. The creative is an image (occasionally a short video) that needs to land in each chat together with a brief caption that greets the recipient by name — e.g. “Hi John, check out our new offer …”. The goal is to do this in one sweep, at essentially zero additional cost per message.

Here is what I need from you:

• A repeatable method, script, or lightweight toolchain that connects my contact spreadsheet (CSV/Excel/Google Sheets) to WhatsApp and broadcasts the image/video with the personalised caption.
• No per-message or per-recipient fees: the solution should rely on the regular WhatsApp/WhatsApp Business account I already use, or any open-source bridge you can configure on my side.
• Throughput must handle ~50 000 deliveries in a single day without manual clicks after launch.
• Simple interface so my team can run future sends themselves (CLI script with README, or a small web GUI).
• Proof of concept: you will demonstrate a test batch to 20 numbers from a sandbox list, showing that the media file appears correctly and the first-name placeholder resolves.
• Handover package: clean, well-commented code (Python/Node/other), quick-start guide, and any dependencies documented.

Your approach could leverage Selenium or Puppeteer automation of WhatsApp Web, the WhatsApp Business client with a local database, or another creative workaround — as long as it keeps costs at zero and respects the platform’s limits. Please outline the tech stack you favour and previous experience with high-volume WhatsApp outreach when you reply.