I couldn't disagree more.
You want your apps and work files on your boot drive. Not only is there no advantage to splitting them but APFS Snapshots allows you to quickly recover if you completely screw up. Ok, I know this never ever happens when editing AV...
Do also get an external. Docks and housings are cheap beginning $29 for USB 3. A raw 860 EVO SSD lists for $349 2T and $799 4T. Considering that 4T is a $2,700 upcharge on an iMac Pro... An SSD in a Thunderbolt enclosure or dock supports TRIM; USB 3/3.1/C etc. enclosures cannot. An SATA drive over USB 3 is only a little slower than over TB, however. Your bottleneck is the 6G theoretical limit for SATA III. Yes, there's RAID and NVMe but, if going there, save your money and get an iMac Pro.
Although I do agree that you should offload finished projects to the external using aliases or symbolic links on your desktop to make them seamless, you do not want to work on a file sitting on an external unless it's a Word doc or spread sheet. Ever. Why? Remember the SATA III bottleneck?
Why would you throttle Photoshop rendering or AV editing to the theoretical max speed of a 2011 iMac—or even slower over USB 3? The NVMe PCIe 3/4 blade in a 2017 iMac is many times faster for these tasks. That's why.
The right size is System+Applications+documents and all work files+the largest video you ever plan to edit+a bit more because you never know.
I'm guessing that, while a 1T blade may be big enough for now, a 2T probably is and it's more 'future proof'. You will not kick yourself later for buying bigger now. OTOH, I can promise that, if you go too small knowing what you now know, you won't recall any money you saved as you wait... and wait...
APFS Snapshots only works on the boot drive. It has saved my butt a couple times since I discovered it.
http://www.motunation.com/forum/viewtopic.php?f=4&t=64689
To quote James Brown: Goodgawd! Yes!
(stinger) Yes!
(stinger) Yes!
(chord)
MyClouds support Time Machine right out of the box. Changing Drives on the Mirror takes one minute and involves a single screw.
The WD MyCloud Mirror:
https://www.wdc.com/products/personal-cloud-storage/my-cloud-mirror-gen2.html
or Expert/Pro:
https://www.wdc.com/products/network-attached-storage.html
The raw WD Red HHD is a low energy, cool running drive with extra heavy bearings and a longer warranty—ideal for Time Machine.
You can often buy used Mirrors on eBay cheap, pop the way too small drives out and drop in a pair of 8T or 10T Reds. I do not keep them at the default RAID 1 setting, however. Instead, I set them up as JBOD so that the individual drives alternate hourly. It's quieter that way.