I'm mostly anxious about somehow losing all my data when I make this transition since I have zero experience when it comes to running a NAS ....

Not understanding how you will set up the vdev without losing data.

Here is my understanding of setting up Truenas.

You have that data sitting on a drive. At some point you will copy that data FROM the drive onto the VDEV.

But.... whatever drive is inside the NAS that you construct the VDEV from - to turn it into a VDEV you are going to need to (effectively) format the drive. You can't get around this. I am not aware you can take a drive with an active partition and data and preserve that partition and data.

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I have a similar but slightly different problem. I have a solution that sucks, but it works. You might be able to adapt some piece of it for your own problem.

I have a 300 tb Gdrive account. Google got wise to me, sent me an email to tell me - and I quote, 'Cut your shit out. Also, we are freezing write access. Have fun. Asshole.'.

(you sort of have to read between the lines)

I have a 40 TB folder that I am desperate to keep. I don't really have 40 tb of free space.

So I am building a NAS. In the meantime I am terrified of Google shutting my account down.

I took a 7tb drive on my computer and purchased a BackBlaze account and did the upgrade to the account where they keep deleted data for 1 year. 365 days.

I read 7tb of that 40tb into that drive then allow backblaze to upload that data. When it is done I delete all the data and do another 7tb.

I can go into the backblaze account and recover the deleted files for 365 days so... the clock is a ticking.

(if you pay attention to the scheme you will see I am after redundancy. If all goes as I think it will go I will populate that folder from the Google Drive account. If Gdrive goes poof (as I fear) I can download it from Backblaze)