Alright!
It took me a while to get all the components, assemble them and then pull the trigger but I have now emerged on the other end, transfer completed and would like to fullfill my promise about a post-mortem report about how the process went and the issues, as well as the solutions, I encountered.
First off, a huge thank you to JonathanM for his guide further up in the thread, it was invaluable.
As for the "nuances and dragons" I encountered along my journey:
After I updated my Unraid version from 6.9.2 to 6.12.4 the GPU passthrough to my VM stopped working. After looking on the forum, this seems to be a wide spread problem with no real conclusive way to solve it. I tried to get it working again for a little while but since I wasn't going to use the GPU in my new setup I let it go.
The motherboard in my new setup did not allow me to boot using legacy mode no matter what I tried (double checking the USB was the first and only thing in the boot order, disabling fast boot, disabling XHCI Hand-of, Enabling Legacy USB Support and making sure the USB is plugged in to a USB 2.0 port on the motherboard). What ended up solving the issue was changing the name of the folder on the Unraid Boot USB from EFI- to EFI to enable UEFI boot, and just like that it started working exactly as expected.
Note that the USB was always visible in the BIOS, but whenever I tried booting to it I was just brought back to the BIOS screen again.
Also, the boot times of my new machine was very slow, even with fast boot enabled, but disabling XHCI Hand-of seems to have massively improved the boot times. Not sure what it does or why, but if you find your new machine slow to boot, maybe disabling this could be an idea to try.
After booting in to Unraid my new drives were initially not showing up in the Main overview. This was due to the fact that they were all plugged into a SATA expansion card and the firmware on the sata controller was apparently old. I downloaded the new firmware and installation software from a third-party site and went ahead and flashed the new firmware to the controller. After doing so the expansion card worked a treat and the drives immidiately showed up in my overview.
To update the firmware on this card you will need another PC running Windows, as the firmware upgrading tool is only windows compatible. Another requirement is that the chipset on the motherboard can't be a 600-series chipset (Atleast at the time of writing). This is the chipset used by the latest 13000 and 14000 series processors from Intel.
If you have a lot of other PCIE cards plugged in to your Windows machine that you want to use to update the firmware on the card you might need to remove some of the other PCIE expansion cards in your system. I had both a USB 3.0 expansion card, an ethernet adapter and a GPU plugged in to my machine and it caused the Sata controller card to not be visible in the firmware updating software, possibly due to there not being enough PCIE lanes on my CPU. To solve this problem I simply turned off the machine, unplugged the USB and Ethernet expansion cards, rebooted and tried again, and just like that the card showed up and I was able to update the firmware. Thank god for this reply from da_stingo on the forum, without it I would have never thought to try to update the firmware and assumed my Sata expansion card was a lost cause.
One last thing to note about this SATA expansion subject. Apparently some expansion cards, depending on the chipset they use, don't play nice with Unraid/Linux. This is something I was completely unaware of and it was only by luck that I ended up buying an expansion card with a copatible/reccomended chipset. If you are planing on doing this, please have a look at this forum thread for reccomendations on which chipsets to look out for before buying.
Slight ammendment that was needed to the great guide provided by JonathanM mentioned above, I was unable to assign the cache pool as the secondary storage and the array as the primary storage for the shares that were hosted on my cache drive so instead I simply kept the cache pool as the primary storage location, set the array as the secondary storage location and just reversed the transfer direction to go from the cache to the array, which yielded the same end result of transfering the files from my cache pool to my array.
Another issue I encountered when doing the moving was that some data did not want to move, namely the docker and virtio folders in my system share, as well as all my empty shares. This turned out to be because I had the CA Mover plugin installed and running. Once I removed that plugin and retried the mover it moved all the remaining files in my system share as well as recreated the empty folders/shares in my new ZFS pool without issue. Thanks to this post on the forum for giving me the solution to this specific problem.
While running mover I encountered some files that had too long filenames to be transfered by mover from my disks to my zfs pool causing some "filename too long" errors to appear in the log terminal. It was fairly easy to fix by opening a terminal and renaming the offending files using the mv command.
Hot tip (that is probably very common knowledge), if your file name or file path contains spaces, just wrap it in "", like this: "/mnt/disk1/this path has spaces/file.txt"
Even after all that I needed to do some manual moving of a few files that were persistently sticking to my cache drive using either mv or the rsync commands. Most of these were temp files in a few different locations that I probably could have done without but I figured since there were only a few folders I would try to get them transfered as well.
And that was that! Over all a fairly smooth transition, all things considered!
Hopefully this final answer can be of help to someone in the future that are planning on doing a similar transition or if someone has any of these individual problems.
I'll mark this final post as the solution so it becomes more visible (Not sure if I can mark multiple posts as solutions) but the true heroes in all of this are JonathanM for the amazing guide he made, JorgeB for general advice and the great list of compatible Sata controllers as well as the other people in the other forum threads that actually had all the answeres. A huge thank you and shout out to them!!
