One way to do it is have a small Python (or any other scripting language really) script that performs text replacements in the Latex source file. This is much easier in Latex because it’s plain text. I don’t know of a solution that doesn’t involve writing your own code (apart from LO/Word serial letters).
Is using Latex an option? I’ve done that and it works quite nicely. You can easily populate a template e.g. using Python.
The UFS storage inside an Android smartphone is basically an SSD. Both use flash memory, so it’s reasonable to expect that they have similar failure modes. But I’m not sure if you could diagnose such a failure on Android.
I have seen multiple (quality) SSDs with unrecoverable read errors after a few years. All of them had plenty of spare area left. Maybe some kind of retention issue?
I’ve also recently built my own NAS and I’ve gone through similar considerations. One of my mayor decisions was not to use btrfs because it’s not recommended for Raid Z1/Raid 5. With that, I landed on ZFS and TrueNAS Scale. Note that RAID expansion should be landing in both very soon.
Things with TrueNAS were pretty easy, very quick, and everything worked nicely. However, I noticed that it was constantly accessing the disks and preventing them from spinning down. I really wanted to keep the power consumption low (<20 W idle), so I eventually decided to just go with Vanilla Debian + ZFS. I can recommend that if you want to tinker with things yourself. Otherwise, I’d recommend TrueNAS Scale.
As for migration, you might be able to create a degraded pool initially, copy over the data, and add the parity disk last. Raid expansion would ofc also help there…