I'm a big fan of the DS1821+ so I would go with the DS1621+ for the 4 core CPU, extra drive bays and normal PCIe slot (and install more memory in it). Though for single thread processes the CPU in the DS923+ is faster. I don't if any part of DSM or Synology packages only use a single core.
I would go with SHR-2 over RAID 6... but they are both overkill for only 4 drives.
I like to keep 1 drive bay empty so I can use the "Replace drive" feature to
quickly replace a drive without stressing the other drives in the storage pool. The empty bay can also be used for testing new drives before using them to replace a drive or for wiping old drives.
Where I live 2.4 inch SSDs cost almost as much as as NVMe drives.
- WD Red WDS400T2R0A 4TB SSD are $1011 AUD
- WD Red SN700 WDS400T1R0C 4TB NVMe are $1121 AUD
If I'm spending $2,020 on two SSDs I might as well spend $2,240 and keep 2 drive bays empty for more HDDs. The advantage of 2.5 inch SSDs is you can create a volume without needing a script. Actually there is a 2nd advantage: if the SSD drives are in bays 1 and 2 DSM will usually run from those SSDs instead of the HDDs.