You are of course correct that with a 3rd party RAID system, a HW controller failure is the weak point. Frankly, I wouldn’t be surprised if it doesn’t take a lot longer than 24 hours to mirror an 8TB drive and during that time you are vulnerable to a 2nd drive failure. Because of that, many RAID companies are implementing the option of dual parity drives so they can tolerate a double drive failure (which does occasionally happen) without losing data. There are actually only a few manufacturers of controllers, but there are zillions of small RAID companies, each claiming some special feature. While a RAID can be more reliable than a single drive (depending on the controller), it is no substitute for a good backup and a 3rd off-site DR copy. Although a dedicated RAiD controller can be faster than a SW implementation by the OS, if it were me and I wanted to mirror (and running Windows), I’d just use the native RAID capability in Windows. Also, some MBs have the capability to implement RAID in their in-board SATA controllers.

I have just downloaded the My Cloud user manual (you have done this of course ?).
Very bad manual and it really tells you nothing because one of the first things you want to know is how long will it take to do things. My experience is that if there is something NOT included in a manual there are only two reasons - the manual writer is pretty ignorant or second its not included because it is bad and they don't want you to know.
Now I am presuming you are talking about a My Cloud Duo which takes two drives. Two drives allows mirroring between two drives. It is called Raid 1. Note this is TWO drives not three.
There will be no communication between two My Clouds to allow them to function together as one device.
I would imagine that a rebuild could take up to 24 hours for an 8TB drive.
Low cost Raid towers are not known for their speed performance.
And you will need two of them because one is not sufficient for proper backup.
Swapping one of the working drives can be a good idea in that it does make sure that the spare drive is still functional. Bad spares do happen. Even just sitting there a spare can go bad. You would be better using the spare drive as an additional 8TB storage thus giving you TWO copies.
A Raid device normally assumes that swapping in a drive is because a drive has failed and that the spare is empty. So it will do a full copy over taking the same 24 (?) hours I mentioned above.
And the problem is what happens if the controller within the Raid box fails ? Then you have lost everything unless you have another copy. And if you are able to source another My Cloud box you need to make sure the box is the same firmware level because sometimes one firmware level won't work (read) with another earlier one. I would want to keep two Raid boxes myself and make sure that drives can be swapped between them (always making sure that I had another copy first anyway).
While Raid boxes are working everything is sweet. Its what happens when the raid box fails that the fun starts.
Relying upon your single Raid box as a form of redundancy is crazy. A Raid box mirroring is best used to allow you to keep on processing in the event of a drive failure with fixing the failed drive in the evening or at a more suitable time. It does have a slightly more obscure function which I never see mentioned here in that a mirrored box can allow some performance gains because while one drive is being accessed for data the other drive can allow quicker access to different data on the other mirror. Unfortunately the My Cloud is connected via a network cable so that might make any performance gains somewhat moot.
Raid, in its various levels, can be usefull BUT you must make sure you have also allowed for the downsides otherwise you might be in for a nasty surprise. Typically at home I would have a lot of trouble justifying any sort of Raid system. Maybe if there were two (or more) users doing heavy processing there may be some justification but otherwise I would stick with 2 single drives and copy between them periodically.

In a commercial environment I have seen so much grief when a Raid controller or multiple drives have failed and it all could have been avoided with proper practice.