> there's not a easy/inexpensive method for me to sign up and join the "bandwidth alliance"

Not unless you run a data center and intend to peer with other bandwidth providers to join your intranet to the internet. It's intended for large service providers like Cloudflare/Backblaze that do direct bandwidth interconnect at physical locations and don't have to involve other intermediaries (like Level 3) to move data between members.

Otherwise you "join" by hosting services/content with an Alliance member and making sure you only use other services that do the same. Even then, bandwidth isn't always free (Azure and GCP still have some costs, for example, but discounted).

If you setup a free cloudflare proxy fronting your B2 bucket, then download from that, the egress from B2 is free because it's going to cloudflare and the egress from cloudflare is free because they don't charge for it.

> there's not a easy/inexpensive method for me to sign up and join the "bandwidth alliance"

As I said, you don't have to. As far as I recall you just use two 'B's in the 'B2B consortium' and it's policy applies to you.