Planning my first Unraid NAS - help appreciated

Greetings,

currently I am planning on building my first ever NAS and I favour Unraid as the OS for reasons I'll explain in a moment. Since I have never owned a NAS let alone configured one I would like to use your experiences and know-how to check if what I want to do is feasable or if I missed something that would make the whole endeavour fail. I tried to read multiple other threads but I wasn't able to find someone that wanted to do the same thing as I plan on doing.

A quick background, you can skip this paragraph if you are not interested:

I have been hoarding diverse types of data for years and amassed a variety of internal hard disks as well as two external USB drives. However, about ten days ago on of those drives (WD MyBookDuo 2x14TB) stopped responding. One of the HDDs makes a clicking noise while starting that stops after a couple of minutes. A quick Internet search leads me to believe that the header is broken. When I attach both drives directly to the motherboard one shows as empty and the clicking one isn't recognised. Since the external drive was in the RAID0 configuration all the data is lost (I know, my own fault; however, it's more of a nuissance than a loss since it was mostly movies, shows and games (ISOs and installers) that I have to recollect, there was nothing vital on the drive).

This is what I currently have in terms of storage:

• 1x WD MyBook 24TB (one HDD)
• 2x internal HDDs in varying sizes (18TB & 12TB)
• 1 (hopefully) still functioning 14TB drive from the WD MyBook Duo
• 2x 2TB PCIe Gen3 NVMe (one is my current system drive)
  1x 4TB NVMe PCI Gen4 that stays in the PC

The first two points still contain data I want to keep but later want to move onto the NAS.

My current plan is to purchase the following:

• 1x 8-bay NAS (haven't decided which one, currently I favour the QNAP TS-873A)
• 4x new 24TB HDD for the NAS
• 2x 8TB PCIe Gen4 NVMe for games/internal storage for my PC
• 1x 1TB PCIe Gen5 NVMe to use as the new system drive in the PC

Here is how I want to set up my system and the NAS:

1. install the three new NVMe drives into the computer and set it up (fresh install of Windows etc.)
2. transfer the data I want to keep on the faster NVMe drives from the HDDs to them
3. install the 4x 24TB drives and the 1x 14TB drive from the failed USB drive into the NAS
   (I am undecided if I want to put the two used NMVe drives into the NAS as cache as one shows "only" 90% health)
4. set up the NAS with Unraid and use two of the new 24TB drives for parity
5. transfer the data from the 24TB USB drive and the two internal drives (18TB & 12TB) onto the NAS
6. remove the two HDDs (18TB & 12TB) from the PC as well as the 24TB HDD from the USB drive and put them into the NAS

What will I use the NAS for?:

Mainly as a storage for films, shows, games etc. I admit to not having used network sharing to stream stuff onto my TV, but with a NAS that option becomes a possibility that I may explore.

With the planned configuration I should have 116 TB of usable space (I know TB ≠ TiB). Even before losing the one HDD in the external drive I "only" had 82TB (and I was a good deal away from using it all up). And if I somehow find myself needing more space in the future I can swap the smaller disks for up to 24TB disks without having to upgrade the paritiy drives.

As you might have guessed, the reason for fixating on Unraid as the OS of choice is its capability to use drives of varying sizes without me losing capacity, unlike in a "classical" RAID (I guess that's where it got its name). At least that is what I keep reading about it.

I know it seems counterintuitive after one hardware failure to put all your eggs in one basket and move all my data onto one single NAS. But if I have to start somewhat anew I want to build it onto a solid foundation to minimise a similar occurence in the future. And I think that a single system with double parity is safer than keeping my data on a multitude of separate drives than can fail on their own any minute. And yes, I know that parity is no replacement for a real backup.

So here are my questions:

• Is it possible to build the Unraid array with the 4x 24TB and the 1x 14TB and put the other drives (24TB, 18TB & 12TB) in later after emptying them onto the NAS and simply expand the array? It should be from what I read, but for my peace of mind I would like confirmation from people that actually have done it.
• Is there something I have to keep in mind when chosing a filesystem so I don't lose capacity? I admit to being a complete newbie and I have trouble keeping all the pros and cons of ZFS, Btrfs, XFS et al. straight.
• Should I put the used NVMe drives into the NAS as cache? Otherwise they would end up in e-waste since my mainboard only has four M.2 slots
• Does anyone maybe have experience with the NAS I favour (QNAP TS-873A)? I saw that it doesn't have a built in GPU so if I need to change the boot order to start from the Unraid flash drive instead of the built-in DOM there seems to be a need to install a GPU into one of the PCI slots to attach a monitor. However, depending on me using the NAS for media streaming that might be a good thing since it would help with transcoding, correct?

Thank you for taking the time to read my long-winded rambling and for any help and/or advice you can provide!