Here are my thoughts:
- InFuse Pro helps in that you won't be doing any video transcoding on the NAS, just dealing with I/O & bandwidth needs. Downside is though that if you are doing multiple streams of different content, that turns the individual sequential read behavior into Random Read behavior (i.e. more disk "thrashing").
- Since you are doing all the editing on local desktop, then we are just talking about storage capacity and transfer time (1GbE vs. 10GbE impact). As 4k usage grows, 10GbE may become more and more important to reduce copy/read times.
- Good plan on backups
Yes for TimeMachine, you can set a capacity "cap" for usage, but that is not a per machine cap. So if you set it for say 1TB, that is 1TB that is shared by ALL time machine users (i.e. 3 machines).
I don't know if browser station is going to remove your VM need or not. If you were wanting a VM to say test website/photo layout views when using IE/Edge, Browser Station isn't going to be able to do that for you as it only emulates chrome/webkit type browsers.
Also in the scheme of system load, the browser (unless you are doing video streaming) generally doesn't add a whole lot on top of the OS overhead. So while browser station might take a little load out of your VM, you are adding additional load in the form of another containerized application (browser station requires container station). Container Station requires minimum of 4GB just to run. Window VM's like minimum 2-4GB (especially Win7/10). So on the smaller QNAPs that max out at 8GB, memory could become more of a concern if you have several larger containers as well as a VM running at the same time.
In your use case, I would go with a 6 bay model. Right now the sweat spot for cost/GB is 4TB, so if you started with say a 3-drive RAID-5 setup using 4TB drives (I personally have been using HGST Deskstar NAS drives), you will get about ~7.5ish TB usable storage to start. As your storage/IO needs grow, you then just continue to add additional 4TB drives to the RAID group. You would get a little shy of 20TB useable once you hit all 6 drives.
At that point you either add an expansion chassis, transplant to a bigger NAS, or swap out for larger drives. One thing to keep in mind on the larger drive swap out, ALL drives have to be larger size before you will get space lift if they are in a RAID-5 group.
As for x53b vs. x73, you get more horsepower and expansion out of the box on the x73. Some of the capabilities you can add to the x53b (m.2 SSD, 10GbE), but there is no option to upgrade CPU, add additional slots, expand max memory limit, etc. If you outgrow the NAS, you are buying another one.
TS-653b w/4GB runs $989ish (amazon us)
TS-653b w/8GB (max memory) runs $1122
TVS-673 w/8GB runs $1149
So at 6 bays the TVS is almost the same price for a lot more box (2x CPU, DDR4 vs. DDR3, 64GB max vs. 8GB, m.2 SATA included, etc.).
At 6 drive bay size, the TVS-673 is a no brainer to me vs the 653b. At 4 drive bay, the price delta is a little steeper and the decision a little more involved.
Myself I tend to go for more horsepower on the HW front as my needs usually grow over time as I figure out more things I can do with it, and you can't easily upgrade CPU/expansion capabilities on the majority of NAS models without swapping the box out.
