I do this all the time for friends' computers I upgrade.
I have an external USB disk cloning device which also works as a regular external disk reader. You can place the original disk in slot A and the new disk to slot B and just press the clone button. The lights will stop when it's done. Note that you in our to clone, you will be a same size or larger size SSD in slot B.
If you decide to do a fresh install, you can use any external hard drive reader or enclosure to attach via USB after Windows is ready in the new SSD. Then you can delete the partitions and format using Disk Manager in Windows.
Hope this helps!
The easiest way I've done it is by using an external USB hard drive cloning device like this one:
So long as you have an equivalent size drive or larger you're copying to, it works great!
I bout with the new drive, then plug in the old drive back in the external unit to format it.
Your hard drive CT1000MX500SSD1 has a SATA connector. Therefore, you need to choose a SATA hard drive enclosure. If your MacBook has a USB-A port, this dual 2.5" + 3.5" enclosure will be just fine. If your MacBook has a USB-C port, you'd better choose a USB-C docking station.
Here is an option:
It's not exactly applicable, but I always love any reason to reference Rule 37.
As I understand it, the GPU can be used for transcoding to reduce load on the CPU. Whether or not this is something you need, I couldn't say (though the answer to question 4 indicates you might benefit). My server is much, MUCH, MUCH less current/powerful than yours. It struggles to transcode/stream more than 1 vid at a time (a friend has figured out anything they want to watch between 5-7pm is going to be pixelated because that's when my MIL watches the news from my antenna). Generally, transcoding isn't necessary for locally hosted content, but I've recently discovered some vids encoded with H265 are nauseatingly jumpy (and these are just standard 1080p, not 4k) when played with the Roku Plex app, so I've had to disable direct streaming on a couple of devices. But the basic answer to the question #5 is yes. As I understand the magic, the client (tv/stream box) tells the server what it can handle, and the server transcodes the file to match.
Yes, the synology would be used instead of your planned server. It has it's own CPU/memory, and there is a plex server build that runs on it.
As for the transfer of the data, if you can get everything hooked up with wired 1gb connections, it would probably take a bit over 20ish hours. If the server supports USB3 you could get something like this. You would have to remove the hard drive from the current machine, but USB3 has a bandwidth of 5gb or more (I think?), so it could cut time dramatically. Downside is if this is your new machine's system drive, you won't be able to use it while copying.