M.2 NVMe for DS420+

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Yes, They are not strict with NVMe.
When choosing NVMe for caching you need to look at reliability ratings such as DWPD/MTBF. Here are a few on the list
https://nascompares.com/answer/nvme-m-2-...dwpd-mtbf/
Caching means a lot of read and write operations, unlike regular storage situation.
You would need to look at models with 0.5DWPD and above

970 pro – 0.66DWPD

Synology SNV3400 – 0.68DWPD

Seagate FireCuda 510 – 0.7 DWPD
Seagate IronWolf 525 – 0.7 DWPD
Seagate Firecuda 530 – 0.7 DWPD
Seagate PS5 Game drive – 0.7 DWPD

Seagate FireCuda 520 -0.9 DWPD
WD Red SN700 – 1.0DWPD

I hope this helps.