> The Tom's review of the revised Crucial P2 500GB was a 2-chip and the QN322 2TB also appears to be 2-chip vs 1-chip for smaller capacities of the QN322, so the performance boost from higher capacity won't be there.

You need to make the distinction between how many packages of NAND flash are soldered to the PCB, and how many dies of NAND are stacked in one BGA package. A 2TB SSD requires more than two NAND dies because the largest NAND dies in production are in the ballpark of 1-1.33Tbit, so a 2TByte drive would typically have 16 NAND dies in total.

> And the SLC cache size isn't fixed but rather a fraction of the free space.

That doesn't stop a 2TB drive from generally having 4x more SLC cache than a 500GB drive, other things being equal.

Yes, data scrubbing (read only) is still a good idea for SSDs. If data in a marginal memory cell is starting to degrade, reading that data will cause the SSD to notice a correctable error and write a fresh copy of the data. Simply leaving the drive on the shelf would put you at more risk of degradation leading to an error that the drive was not able to correct (though a RAID layer above the individual drives may still be able to recover the data).

Many drives will do some amount of background reading on their own to scan for degraded data. But this isn't visible to the host system and not guaranteed, so it shouldn't be relied upon and doing some extra reads (eg. once a month) is harmless.

I strongly believe QLC SSDs are a great choice for NAS usage, unless it's very frequently modified data. It's still more expensive than hard drives, but the noise, power and performance benefits can be worth it.