When you say most accessed, do you manually assess this or is some programme working this out for you?
I store stuff like home drives, documents and installation binaries on the NVMe hosted volumes. My media sits on HDD. To be honest, this has more to do with the size of the volumes as it does with latency or speed of access (the reason for the wait for 2TB NVMe drives is that I have over 1TB of FLACs)
I actually encourage HDD spin down in both my NAS. I have a primary NAS (DS920+) and a secondary NAS (DS420+). Both NAS identically sized HDDs and NVMe drives in them. Each drive hosts a single volume (I do not use RAID), each volume hosts one or more shares. The shares are synchronized from the primary to the secondary NAS once per day. Important data is also synchronized to cloud storage and occasionally to a USB drive which is kept off-line at other times.
This setup has the following benefits...
1. Single drives (i.e. non-RAID) can spin up and down as and when required saving power and runtime in both NAS.
2. I don't need to worry about getting the right type of drives for RAID (CMR vs SMR) - I can buy the cheapest possible drives from a £/TB perspective, this usually means shucking a drive from a USB enclosure.
3. Upgrading is cheaper, I don't have to replace a whole set of drives. I can just add a single drive to each NAS. e.g. at the moment I have the following drive config... 512GB NVME, spare M.2 slot, 4TB HDD, 6TB HDD, 6TB HDD, spare HDD slot. To upgrade media storage on HDD, I will put another drive in the spare slot in each NAS (probably an 8TB or 10TB), I will then migrate the share(s) from the 4TB drive to the new drive and retire the 4TB drive (which will be 4 or 5 years old by this point). This will free up another HDD slot for the next upgrade.