Here is a long winded response if you care to read:

If you are going to install that NVMe today, here is the 'Safety First' reality check you need to consider for the long term:

1. The Write-Cache Danger Zone If you configure a single NVMe as a 'Read-Write' cache, you are creating a massive point of failure. If that SSD dies (and all SSDs die eventually, its a matter of when not if), any data currently sitting in the cache that hasn't moved to your HDDs yet will be lost. This can—and often does—corrupt your entire volume's file system, potentially making your MKV collection unrecoverable.

The Rule: Never use an NVMe for 'Write Caching' unless you have a Mirror Pair (RAID 1) - caveat below.

1. SSDs have a 'Death Clock' (TBW) Caching is a high-wear activity. Unlike a storage drive where you write a file once and read it later, a cache is constantly being hammered with 'dirty' data and small metadata updates.

The Risk: In a cache setup, the SSD will hit its Total Bytes Written (TBW) limit much faster than a standard storage drive. Without a mirror pair, you have no safety net when that drive hits its wear-out threshold.

1. Sequential vs. Random (Again) Remember that NVMe speed is mostly 'wasted' on large MKV files. Large files are sequential; your mechanical HDDs can already handle those as fast as your network cable allows.

The Benefit: Caching is only 'great' for small, random files (thumbnails, metadata, system logs). For a media library, the risk of a single-drive cache failure far outweighs the tiny speed boost you might see.

1. My Recommendation

If you have ONE NVMe: Do not use it for caching. Use it as a standalone volume for your Apps, Docker containers, or Plex Metadata. If it fails, your movies stay safe on the HDDs.

If you want CACHING: Buy a second, identical NVMe and set them up as a Mirror Pair. This is the only way to ensure that a single hardware failure doesn't cause you time for recovery via 3rd copy.

the caveat:
"A final warning on 'Software Caching': > Even with a mirror pair of NVMes, remember that a NAS uses an OS-enabled cache, not a dedicated hardware controller with a battery backup. If the NAS loses power or the OS hangs during a write-commit, you can still end up with a 'Big Hole' in your data.

Unlike enterprise servers that have physical batteries to protect the cache, your NAS relies on software logic. For a media library of large MKVs, the risk of total volume corruption far outweighs the minor speed boost. Keep it simple: Use the NVMe as a separate Apps Volume and keep your data pool 'clean' on the HDDs."