Synology at the moment only try to sell their (Toshiba) drives to be used on XS series models.
Plus series models will work with Ironwolf or WD Red plus drives.
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
Seagate FireCuda 510 – 0.7 DWPD
Seagate FireCuda 520 -0.9 DWPD
Seagate IronWolf 525 – 0.7 DWPD
Seagate Firecuda 530 – 0.7 DWPD
Synology SNV3400 – 0.68DWPD
WD Red SN700 – 1.0DWPD
I hope this helps.