Given I just set this up successfully with cloudflare, I thought I’d share my experience. Cloudflare is not your traditional CDN. I’m using it for DDoS protection, SSL, and caching. However, to setup S3 storage, it seems you do need another CDN. I used BunnyCDN which is relatively inexpensive. You setup a pull zone that points to your upload bucket url (created in backblaze).
Then just follow the instructions here: Configure an S3 compatible object storage provider for uploads - #106
Note this only works for future files and images uploaded. It doesn’t transfer your old files.