Welcome! Let me give you my thoughts on this.
Get as high powered a NAS as you can. The one you are considering may have a nice price point, but it only has a Celeron processor. It is not that great in speed and what it does. If you are ONLY going to do file serving, it might be OK. But you want to use Plex and you’ll probably find other apps you want to use as well. Plus doing things like backups, PC syncs, etc. all take resources.
I’ve had a TS-451 for several years. It is way underpowered (and I only use it now for my security cameras). Almost a year ago I purchased a TS-873A. Much more powerful, but I wish I would have spent another $700 to $1000 and bought a more powerful unit. I don’t know your economic/financial situation, but buy as big/powerful a NAS as you can afford. You are effectively buying a server. You don’t want your server being less powerful than the machines connecting to it - right?
Also, get as much memory as you can afford as well. At least 32GB. I have 64G in my TS-873A and 32G in my TVS-672XT (which I found used on eBay as a great deal). QNAP says you have to buy their memory. Balderdash. It will cost you as much as a NAS from them! Brand name memory of the proper speed/type will work fine.
Since you want to run QuTS Hero, you will DEFINITELY want a more powerful NAS and lots of memory. I would not run Hero on a Celeron system. Not no way. Not no how. ZFS does inline data deduplication and compression on the fly - so again more CPU usage. Also, the way space is allocated and shares are created may surprise you a bit - a shared folder is treated like its own drive. This is very different from the QTS OS. ZFS has some good advantages to it like WORM. But it has some weird quirks where you can get unrecoverable file system errors. I just went through this. My NAS seemed fine but a QNAP tech showed me the issue and it could have gotten serious. I had to basically wipe the thing and start over. No idea how these errors happened - but they did. But I have Hero on both my larger NAS units and I generally like it.
OK, now on to some of your questions. You can set up different user accounts where those accounts will have dedicated access to their “home” folder that is just theirs. You can set up privileges of what folders users can access, what apps they can access, etc. So you can have that segregation easily.
You can mount your cloud service as a volume on your NAS. There are two ways to do it using an app called HybridMount. First is just as a remote cloud mount. That mounts the cloud drive as a volume the NAS can access. However, it does not share that volume with your network (ie: You won’t see it on your PC/Mac as a shared folder you can access). You can access it from File Station in the NAS web interface.
The second method is called FileCloudGateway. In this method, you create a shared volume on the NAS for the cloud data and it synchronizes it to the NAS and vice versa. It basically caches frequently used data locally so you have access to it faster. When it works, it’s pretty cool. I had major problems getting it to mount a WebDAV volume on my web server and gave up. But I’ve also used it for DropBox, etc. This method DOES share the cloud service as a shared folder that other machines can easily access. You get two free licenses for File Cloud Gateway. If you have more than two cloud services, you have to buy an additional license for each service.
Can you “clone” your Android phone? I wouldn’t say clone, but you can use a mobile app called Qfile to sync photo albums, music, documents, etc. It works pretty well. And QNAP has relay services you can connect to when you are away from home so you can always access the files on your NAS wherever you are (best way and fastest is to set up a VPN to your home network but QNAP’s service is not bad).
As far as cloning your laptops, there’s a number of backup programs available. I’m a Mac guy and use TimeMachine backups from my Macs to the NAS units. I also use Qsync to synchronize important files that I want to access from every machine. If you are a PC person, there are a number of PC backup apps that work with the NAS.
There’s a lot of info for you!
Please reach out if you have more questions or want to learn more.