@robyt Can you post a screenshot of where you see NBD? I don't see it anywhere in mine but I'm not sure if I'm looking at the right place.
I think you need to get things working first and then start trying to push the envelop. I would set it to 1 and see what your speeds look like. Then you can slowly increase the number of connections until you stop seeing speed increases and/or you see http connection timeouts.
I'll admit that I haven't been paying attention to the whole thread, but I'm not sure what more testing you feel you need in order to determine that the problem is your NAS. What is the goal you're attempting to achieve that you're concerned that different hardware wouldn't solve?
I'm not sure what your configuration is (if you've posted it already, my apologies), but posting your XCP-ng host(s) details, XO specs, QNAP specs, and what other duties, if any, the QNAP is performing, that would be helpful. They make some powerful models, but most of them don't have that much oomph and can really only do one, maybe two things at once. And this goes doubly so if you're using very large VM disks instead of network storage.
In my case I'm backing up to a fairly beefy TrueNAS running a 11z3 setup on a 10G network. My VMs are all configured with smallish disks and pull the majority of their data from the same NAS. My biggest bottleneck is that my XCP-ng hosts only have 2.5G NICs which is why I'm considering moving XO to something with 10G in order to support the fully bandwidth.