Steam uses a certain amount of disk per second. The average is over 30 mb per sec. It seems like the download is reliant on on how much disk is being used. Whenever the disk lowers the speed lowers with it and the download frequently stops and starts. I am wired to a good extender. I usually dont have problems downloading on epic and ect
Steam is extremely strange. I have one of the faster NVM m.2 ssd. It still get stuck at time for no real apparent reason. My theory is that there are a bunch of very small files that were sent and it is taking time to unpack them as they are written.
Generally you if you check the event log it will show errors writing to the SSD if that is the cause. I bet it is just steam software issue.
Maybe try to download a windows 10 image from microsoft. This is a like a 8gbyte file and I have seen people download it at full gigabit speeds so microsoft has lots of bandwidth. It is not so much how fast it run it is does it run consistently. You can see the rates in the resource monitor under the network tab.