There are lots of places on the Internet that try to do what’s being tried here, if you want further opinions. Discussing here is more likely to find things that work with Duplicati, of course, and that counts for a lot.
Once something is proposed, forum search (upper corner) can see if anyone not right here has used it.
One thing that’s changed recently is B2 now has S3 compatible APIs which possibly make a difference. Original B2 API (the one that Duplicati calls B2 Cloud Storage) steered client to use specific equipment.
Duplicati now offers two different S3 compatible implementations to select from, and speeds might vary.
If you get into S3, Wasabi prices might land similarly to B2, after factoring in B2 download fees, although Wasabi has other price adds like minimum storage retention policy. Going even further is Google Cloud, with what “looks” like an incredible deal for “Data storage” in the “Archive Storage” class, but there’s the “Retrieval and early deletion” fee to consider. They do have three other storage classes to choose from.
Note that Duplicati ordinarily does do downloading during compacting to prevent wasted space buildups. This can bother Amazon cold storage for technical reasons. With Google it might just impact the charge.