Good things take time. Your team is doing god's work. Remember to stay healthy, in both mind and body!

It looks like this issue occurs only when saving the backup to non-block storage. When saving to block storage ("Store backup as multiple data blocks instead of a whole VHD file: ON"), it does not, at least in my environment.

I second 4 being a good sweet spot, I personally have seen very little improvement from going higher than 4 in my environments.

Any of the host: if they are in the same pool, that's logical. Only the master is needed to be reach. XAPI will probably be in "Starting state" as long as all SR aren't plugged. If you have the SR on a VM on another host than the master, reboot the master only, you should be able to connect sooner Alternatively, check https://docs.xcp-ng.org/troubleshooting/

You do not enable NBD in your VM, you enable NBD on a XCP-ng network, that's it. There's no drawback to enable it (well, it's a new service exposed but it's OK)

You can with the JSON-RPC API. The REST API is still very young, and we only started to modify objects during January release.