Another thing is that unless you have a very specific need for SSDs (such as heavily random access focused workloads, very tight space constraints, or working in a bumpy environment), mechanical hard drives are still way more cost effective for storing lots of data than NVMe. You can get a manufacturer refurbished 12TB hard drive with a multi-year warranty for ~$120, while even an 8TB NVMe drive goes for at least $500. Of course for general-purpose internal drives, NVMe is a far better experience than a mechanical HDD, but my NAS with 6 hard drives in RAIDz2 still gets bottlenecked by my 2.5GBit LAN, not the speeds of the drives.
Don’t forget about power. If you’re trying to build a low power NAS, those hdds idle around 5w each, while the ssd is closer to 5mw. Once you’ve got a few disks, the HDDs can account for half the power or more. The cost penalty for 2TB or 4TB ssds is still big, but not as bad as at the 8TB level.