My data does not even live on the same device as my Windows OS. At least most of it. This makes swapping between PCs and Backups just easier.

I must say WIndows works mostly fine for me. Yes it definitely has some bugs but the last time I had to reinstall my own machine was in the Windows 7 days. Depends also on what hard and software you use though and how well versed with troubleshooting you are.

I mean for most people this should work fine but it seems they have no initial cap. So it is pretty likely it will take days or even weeks for many people to fully transition (or years when you are a data hoarder). Also I think a weekly or monthly limit would be better as it is pretty likely IMO that will occasionally need more than 50GB even as a rather casual user.

Newer generation Intel boards should be quite a bit better than my numbers, too.

A modern, barebones board with no extra cards, 1Gbe and an NVMe idles at 10W with an ATX PSU. A dedicated NAS can not really do a lot to lower this a lot.

I am not really sure if your strategy is the best one. How many movies do you actually have. 3TB can be ~3,000 ones in low quality or ~50 4K remuxes. for a few dozen movies and a handful of duplicates, your solution is overblown.

What I would do:

1. Copy each of your drives onto your new main drive. Keep them in separate folders that are named in a way to indicate where they came from. File sorting tends to be messy and mistake prone so do not delete the originals right away.
2. Make sure your naming is correct. Plex works only great if you stick to the naming conventions. Using a tool like tinyMediaManager or filebot can help you a lot in this regard.
3. Now it’s time to check for duplicates. I somewhat doubt that the issue is so big that it is really worth it to automate this. Yes, MediaInfo is a great tool for this although you should look up a bit how to actually interpret what the program says. There is some correlation between file size and quality but a lower bitrate reencode with a more modern codec or a slower encode can look better than a high bitrate, old one. And you might care about stuff like 5.1 sound, subtitles, non dubbed etc
4. Setup a program like overseer. This makes managing your library easier
5. Now you can check your stuff for quality and missing features and note down your issues in overseer to deal with them later. It is not possible to automatically determine if an encode is low quality or not in many cases. When CRF was it can be done to some degree but for bitrate encodes it is kinda of impossible. It also depends a lot on your preferences what can be considered “good”. I like to keep the film grain or noise, others actively avoid it. As noise cannot be compressed well it tends to blow files significantly though

And this is why you make sure you have a working backup right now. Not next week. Not when you have some downtime to set them up. It is way too common that something like this happens right before you planned to back up everything.

321rule, automated, snapshotted. Cold backups on top for some edge cases.

File name extenuations are much less magical than some users think. YOu can change them to whatever you want. This movie was made in 2006 and something went wrong while renaming it. Should likely be Idiocracy (2006).xxx. You can determine the correct extension by using a tool like MediaInfo although most video players will play it regardless as long as it is a video file type.

But the power draw is much higher and those servers are not exactly quiet. A single modern i3 is faster and comes with a great iGPU for transcoding.

And I somewhat doubt it will actually solve OPs issue. If the workload is too high for a 1070 just adding a bit of CPU will not help. I guess they try to transcode 4K HDR content. This needs either modern hardware or a lot of brute force (17,000 passmark score per stream)

Would like to run raid6?

RAID z2

For a Plex server I would almost always opt for something else than ZFS. The benefits are slim at best and being able to easily and quickly upgrade your capacity with some random drives is much more useful IMO.

I’d also like to set up an FTP on this server to make it easier to transfer files between remote locations.

Might want to secure that

Definitely. Just run your own VPN. And I would likely opt for Rsync or something similar.

Why input meta data and the like yourself? Tartube and tube archivist already solved this problem.