aetherspoon

Where are you seeing performance issues? As your issue might be less with your CPU and more with storage or some other bottleneck.

Depends on your gaming box.

If your gaming box has fewer cores (say, an older i5 or current i3 CPU), you might notice Nextcloud running. Pihole is so light I doubt you'd notice even if your gaming CPU was a potato.

If your gaming box is more than four cores, I doubt you'd ever notice.

Out of curiosity, do you have everything in /tank? As in tens of thousands of directories or files directly in the root versus a tree of directories?

Around 90-100W typical for the part of my lab actually powered on. That is my NAS (60W typical) that has five 12/14 TB hard drives attached to a C2750 and my VM Host (35W typical, but bounces between 20W and 40W), which has a couple of SSDs attached to a R7 1700.

1. RAID-Z2 or RAID-6 is probably more than sufficient, as that has two parity drives for a pool of drives. RAID-1 does make more sense if you are only using two drives though.
2. It can work, but generally USB enclosures aren't the greatest approach for what you're trying to do. Do you have space for your two internal drives instead?
3. I mean, it is exactly that - a filesystem. It is a rather advanced one, and it does have some features that would be useful for you, like snapshots so you can go back after messing up a file. It also tends to run fast at the expense of it being a bit hard to expand and eating more RAM than you'd expect.
4. Generally, anything but an SMR drive.
5. I'm not quite sure what you mean?
6. PCPartPicker, sort by price-per-TB?
7. There are only a limited number of RAID configurations, if that's what you mean?
8. It depends; are you disciplined enough to actually run backups, or are you a normal person that needs automatic cloud backups? :P But seriously, do you think you'll run manual backups often enough?
9. ZFS and btrfs both do a decent job at handling such things.

I mean, a NAS is literally Network Attached Storage. Your old laptop has storage and, presumably, is on the network; that's a NAS.

The reason why people have standalone NAS boxes is because a laptop usually can't hold all that much in the way of storage. My NAS has 42 TB of addressable storage; that's not really viable on a laptop. Add in any form of redundancy (my 42 TB of storage comes from five hard drives), caching (32 GB of RAM helping with a read cache), or other services and people quickly outgrow a laptop or even a miniPC.

I'm generally of the camp that only have storage and storage-based services on my NAS, so the CPU of my NAS is super weak compared to my actual home server. There is a good chance the CPU in your laptop might be stronger than my NAS's CPU even. Other people combine their NAS with their home server, needing a stronger CPU as a result.

As for why a prebuilt? Some people don't want to delve into that and just want Storage That Works (tm). I don't dive into networking content all that much, hence a prebuilt router instead of something using opnSense or something. I'm happy playing around in the guts of a storage box (it really isn't all that complicated), so I roll my own.