Is it possible to run WindowS 11 as a virtual machine on a ds1621+ the reason I ask is that I see windows 11 check for a tpm chip.
Before you choose a NAS, you need to make sure its CPU support VMs.
In Synology range, those will be all plus series and XS series NAS with Celeron, Ryzen or Xeon CPU.
In order to check if there is an encryption engine, this will indicate that likely TPM is supported. All NAS nowadays come with AES-NI encryption chip. This can help to identify if your NAS is compatible.
I hope this helps.