You can swap the hard disks individually and hot swap is also no problem. Writing is about 300 to 400 MB/s and reading about 500 MB/s.
Here is a YT video of a Mac user:
https://www.youtube.com/watch
However, such exotic products are very difficult to recommend, because OWC is really not a NAS provider. I would buy a real NAS, Synology NAS are in my opinion the best and easiest to use NAS systems. One with 4 drives, where you can swap each disk if necessary or expand the NAS. It is not optimal to rely on exotic products, even if they look great on paper, because there is less support for them. And there is no community to help you.
If it comes down to pure speed, I would choose the Synology DS1522+, it has 5 hard drive bays that can be installed and removed without tools, can use 2 x SSDs as accelerators for read and write and you can also use a 10 Gbit Ethernet interface in the system, it also already has 4 x 1 Gbit/s interfaces and is practically inaudible in normal operation with the two built-in fans.
Synology DS1522+ (0 TB)