When using the TS-x72 as the model, QuTS Hero doesn't need a license.
I used to run the TS-672 with QuTS Hero virtually within proxmox and it did run stable. The only reason I stopped using it was because I could only get the disks passed through and recognized through the /dev/by-id/ method and not by passing through the whole controller, which is what I preferred. When passing the whole controller, either on a Q35 or i440fx VM, the Qnap wouldn't recognize the disks.
Also, using virtio drivers for the NIC resulted in a working system, but the dashboard and resource monitor wouldn't show any stats for the NIC. No biggie, but needing to use e1000e and disk passthrough instead of more native solutions like sata passthrough and virtio within proxmox felt like a bit of a "half baked solution".
But to recap a long story, using a ts-x72 model would result in a working QuTS Hero (updateable) without license issues.
MODEL_TYPE="Q0121_Q0170_17_10" PATCHED_FIRMWARE="TS-X72_20230927-h5.1.2.2534" DOWNLOAD_URL="https://eu1.qnap.com/Storage/QuTShero/TS-X72/"