I don't know what Cloudflare's economics are like, but R2 egress is free. You pay for storage and $0.36/million for cache misses, which should be a fairly small percentage.
If I had to build a public artifact repository today, that's what I would use. Apparently Maven Central is built on S3 and Fastly. We don't know the terms, but I'm guessing that is more expensive.
Hey, what if we had a Maven tax if our repo was in Java, and an NPM tax for repos in NPM, for a centrally-managed repo with vulnerability checks?