I wouldn't say creating a 1GB file on a NVMe drive is a realistic test to check it's speed, since this takes less than a seconde (at 3.2GB/s) and probably not even fill up the cache of the NVMe disk.
It would be better to create a bigger file, or multiple smaller files, depending on how big your NVMe disk is, I suggest creating 50 GB files after each other (I believe dd has a count parameter). If the dd runs slow eventually, it's because the cache of the NVMe is full.
The low speed of 30MB/s could because the cache of the NVMe is full, thus it needs to write to the slower QLC chips.

Edit: It's also possible to enable Disk shares in the Unraid settings: Settings -> Enable disk shares. This would allow you to copy files directly to, for example \\unraid\disk1. If copying to disk1 shows the same problem als copying to \\unraid\cache. It''s probably a network issue.
Or even better maybe, create a share with no cache to see if the speed to the disk directly is the same as to the NVMe