What will you be using the server for? I don't think you can really go wrong with what you're suggesting, some nvme storage is always nice for running docker/vm and a handful of other uses.
I think it really depends on what SSD's you're using in your pool, what SSD you'll be using as a cache drive and what you use the NAS for. If using SATA/SAS SSD then an NVMe cache pool could provide faster speeds for any task that demands it.