I was going through the support ticket which got closed.
as per support notes, the issue was related to network and after implementing a one-way firewall backups are better.
if you think still the issue hasn't resolved fully, please re-open the support ticket.
we will work through support.
in general, your environment configuration seems to be good enough.
throughput low could be because of the reason which you mentioned always backing up as full.
again we need to check the issue why it is treating as full.