I am referring to the DS1821+, which I have been using for just over a year.

I wouldn't mix HDD and SSD in the same storage pool, even if that would be possible in principle. As the device manages the data itself, you have no control over where the files for video editing or similar are stored.

However, an SSD can be assigned to a storage pool as a cache medium. This means that the frequently available data is made available more quickly. As the media can also be assigned to separate storage pools (SP), it would also be an alternative to create a "slow" (SP) with HDD and a fast one with SSD. Volumes are then created on the respective storage pools.

An expansion with additional media (in any empty slots) is possible at any time with SHR. As far as I know, there are also expansion boxes that can be connected via the eSATA interfaces (2 pcs).

I am not currently using an SSD, so the above considerations are theoretical and should be taken with a grain of salt 😉.

You can compare both NAS systems on the Synology website and see the differences. But the biggest difference is probably that the 21+ supports SHR, the 23xs+ does not and that, above all, no normal storage media can be used in the 23xs+, but that it must be Synology HDDs. That alone costs a lot of money. Especially as 10Gbit via RJ45 only draws considerably more power than 10Gbit with SFP+ if you retrofit a card, which works well.

The DS1823xs+ is overkill for most SMB or home users. I would go for the 1821 - disadvantage: it is already three years "old" because Synology's wild product policy is once again taking ages to bring the SMB product line up to date.

Basically about the performance of classic HDDs:

Never mix HDD pools if you want good performance - HDDs scale wonderfully with the size of the pool in read/write (i.e. from 3 discs with approx. 200 MB/s you get approx. 50% of the combined performance, roughly speaking, assuming conservative assumptions, i.e. your HDDs read 3x200 MB x 0.5 = 300 MB/s each should be possible. It is not possible to say exactly, but experience with Synology shows that RAID 5 scales well if all discs are the same on the hardware side.

Mixed SHR (Hybrid Raid) is practical if you have X different discs, but I would not recommend it if you manage to install at least 3 equivalent discs in a RAID first.

SSDs are only useful in a RAID or in a pool if you have asymmetric data access - i.e. a database in the high-end range with dozens / hundreds of simultaneous users. The 1821 offers the option of installing classic 2.5" SSDs or, if you don't want to intervene in the system itself, you can use the official Synology NVMEs - the model has 2 cache NVME slots, which could probably also be used as a pool via a software update.

You normally edit video data at the editing suite - i.e. this workstation requires reliable NVME. For the archive, a RAID5 (or 6) network and at least 5Gbe-LAN (better 10GBE) is probably sufficient - always with backup, of course, as the valuable data is quickly gone ;-)

So much for my 20 cent.