Ok, so I'm testing a pair of SSDs via a USB-C external enclosure on my 2019 iMac. The speed test should be roughly comparable to what they'd be if they were an internal drive thanks to the nature of USB-C. Here's what I get:
The OWC Electra 6G, formatted in HFS+, gets write speeds of about 265 MB/sec and read speeds of about 503 MB/sec. I then reformatted it using APFS, I get write/read rates of about 285/497 MB/sec. So slightly faster write speeds but about the same read speeds just by changing how it's formatted. Using APFS (encrypted) and I get write/reads of 148/404 MB/sec. Pretty significant and expected due to encryption. I should point out that this drive may have a failing controller since the Mac mini I took it from was having issues waking from sleep and sometimes rebooting and failing to find the drive. So these results may not be wholly representative of a new drive. Its write speeds are supposed to be "up to" 407 MB/sec, but it is falling well short of that.
Using my other WD Blue SSD, again on my 2019 iMac using a USB-C enclosure, formatted in APFS, and I get write/read rates of around 440/480 MB/sec, which is pretty close to WD's best case rates. So I think it's fair to say that the write speed limitations you (and I for that matter) are seeing is perfectly normal and just a function of the slower hardware in the Mac mini.
EDIT: I just wanted to link to a review on Tom's Hardware that was one of the things that convinced me to go with the WD Blue over the other options. They thoroughly tested the drive and were very satisfied with the performance.
WD Blue 3D And SanDisk Ultra 3D SSD Review - Tom's Hardware | Tom's Hardware