Trying to build a 10Gbe network for a client

Hello Redditors of networking!

Let me start by thanking this wonderful community for any input/suggestions/corrections. I am not a total neophyte when it comes to networking, but I have not had enough experience with 10Gbe networks outside of the datacenters I work at. As a field engineer I am only involved in the racking and cabling as deployment is above my pay-grade.

That being said, let's get to the technical stuff!

I have a client who says his graphic designers are complaining that the transfer speed to and from the file server sucks.

The client:

-17 graphic designers

-File server is a DiskStation DS1817+ w/ 4 x 4TB Iron Wolf Drives

-17 iMacs with 1Gb NICs connected via unshielded CAT6.

-Netgear 24-port Gigabit switch

-Average file is around 2-6Gigs.

-Average file transfer speed is around 5-10MBps.

The solution:

I offered them a 10Gb solution by preforming the following (please correct me if I'm wrong):

-UniFi Switch Model: US‑48‑500W + 2 SFP+ Copper modules

-Synology Dual-Port 10GB SFP+ PCIe 3.0 X8 Ethernet Adapter (E10G17-F2) connected to the two SFP+ ports on the switch for a bonded speed of 20Gb.

-17 Dells - (This upgrade has less to do with networking and more with performance, but the NICs are upgradeable to 10Gb nics, and the client approved, so I figured why not?)

The confusion:

1. To create a fully 10Gb network I will need a 10Gb switch that can do 100/1000/10000 on each port individually. However, since the files aren't HUGE, maybe the switch I ordered will be okay?
2. CAT6A is the standard for 10Gb. The CAT6 runs I have are about >= ~25 meters from the switch.
3. Since the file server will be aggregated to 20Gb, and the ports on the switch are 10/100/1000, will the Dells finally be able to use a full 1Gb and communicate with the file server a whole lot faster?
4. I did some math, in theory, if all 17 computers are trying to transfer a Gig file at the same time, with max 20 Gb throughput, each machine should average out to ~150MBps?

Any input will be greatly appreciated. Thank you all in advance.