I'm using it.
I select local path: main-folder/cloud-folder/
Remote path: root folder

So fast you want to go where?

Which OS you using for the build? Got running any dockers?

Finally Drive client v3 released for mobile phone. Still in beta though but so far all usable.
https://www.synology.com/en-global/dsm/feature/drive/app



Problem I encounter is the app takes long time to load thumbnail. Also the usual "press back again to exit" sometimes not working.

Previously v2.5 the Sync Tasks sometimes will fail to complete the sync. I have to delete the task and create the task again. Got 1 time it wiped out all the files in sync folder in my phone, and it show there's no files in the Drive. Luckily in NAS the files is still there, after re-create the task it sync those files back to my phone. So far with v3.0 I haven't encounter such issue, and no fail sync yet. I didn't use QuickConnect, only use VPN.

Not the package in DSM, I'm using the one built-in on my router.

There's security risk when you have opened port (for torrent) and running 24/7, especially more on Windows machine. If there's exploitable vulnerability and virus or malware manage to infect the OS, high chance all your files is encrypted or gone. Windows security have gotten a lot better than ever but one day another Wannacry or Petya like attack might happen and can cause you to lose everything.

I suggest running torrent on isolated machine, such as Raspberry Pi or if your router have such download manager. If anything happen, only files in this machine will be lost. For media server, custom built PC with low power consumption specs will be ideal for 24/7 and that many HDD. Many free NAS OS such as OMV or TrueNAS can be use, it will not only give you media server function but you can use it to make backup of your PC, run personal cloud and act as server for self-hosting service.

I setup settings for my NAS in way that other device (PC, smartphone, tablet) only have read access, so if any device is compromised, it can't encrypt or delete files on my NAS. Since I also running personal cloud, all my device have read-write access only to cloud folder, which have automate backup and snapshots as protection.

It was not super hard. There's some guide online how to configure it. Running through VPN, can maximize the upload speed. QuickConnect speed is capped unless you open port to NAS for direct connection but that's risky.

If you're building new system with OS you not familiar, better to get some other HDD first and tinker with the system until you sure what you're doing before moving all your files to the new system. I didn't use TrueNAS and ZFS but seem there is import disk function which can import data from drive with NTFS filesystem to ZFS pool in TrueNAS. So you will need 1 or 2 new drive and then move each data before format and reuse the NTFS drive.

I don't think the router CPU can handle that much active torrents or files
Router CPU is low power low processing type, plus it have limited memory. Few active torrents probably the max. You need to attach a HDD/SSD or thumb drive to the router, this will be used for the download manager. On NAS side, you can create a remote path to the router storage so you can copy or move files into NAS.
If you always have a lot active torrents, doing it on the NAS would be better as it have better hardware.

Try contact Synology
https://www.synology.com/en-uk/company/contact_us

Synology warns of critical Netatalk bugs in multiple products

I before this haven't got 7.1-42661 auto update. Still on 7.0
After seeing this news, manually download and straight update to 7.1-42661 update 1.

Why not just continue running and see how long until it die? Or already have a replacement?

If you just need a storage space that can be accessed from network and no need for other functions such as backup or RAID, most routers nowadays can do Media Server and SMB. Just attach external HDD to the router through USB and configure in router settings. Your cost will only be the HDD. Get 2.5" external HDD as the USB port might not have enough power for 3.5" external HDD.

You should check what causing the stutter. Are the desktop connection is 1gbps, Shield using cable too? If 2.4ghz wifi the speed might not be fast enough for 10/20gb video file streaming. I don't know if Shield can or not decode using hardware, if use software it might not be powerful enough to decode realtime.

That's cheap. Most other brand more than RM500 for 2 bays.
If you not using it over the internet, make sure to disable any such functions, example TNAS.online service and don't port forward NAS IP on the router.

If the NAS only accessible by using VPN when you outside your home network, then it is correct.

X300 similar like WD Black series, more on performance, not necessarily just NAS.
Other 2 is for NAS use.

IINM, WD Red Pro also CMR. Normal Red is SMR.

Only this 3 brands that are still manufacturing HDD.

Possible. The second disk have Current Pending Sector. See after the number decrease if it will operate back as normal.
https://kb.acronis.com/content/9133

Multi-Zone Error Rate also could be indicative there's something wrong with the surface or read/write heads. If you have redundancy, can just left it like that and see if it fix itself. If no then backup your data.
https://kb.acronis.com/content/9136

5 years is the average lifespan for NAS HDD.

Why using the PCIE SATA card is not a good idea? Don't have a custom built one so I am curious why.

My APC UPS suddenly dead, not sure what reason. Board fried probably, got some burnt smell.

Anyway, got Rightpower F800P as replacement. The USB data can work with my Synology NAS. Can monitor the battery % and send signal to turn off the UPS.