I would not suggest using cloudflare containers for ffmpeg. Feels like it is the nost expensive product they have. Queue to aws spot instances or just have dedicated hetzner machine
This person is only running it for a few minutes once a week or so. Any pay-by-the-minute VM is fine for them.
This is the perfect use of cloudflare containers. Rare, bursty workloads that fit around regular use of Workers.
I use them to generate thumbnails when uploading large photo files for my personal flickr clone, not even noticeable in billing.