NAS used to be the only way to get reliable large amounts of storage, even if one didn’t need the networked capability.
Nowadays, there are a lot of solutions for high-density direct attached storage, so you really need to consider whether you need the NAS features to justify a NAS.
The docker/virtual apps feature of Synology is nice, but if you really want to run virtual apps, you would be much better off getting a high performance cheap computer or running parallels or VMWare (or even free VirtualBox) on your Mac directly (if you have the memory and CPU power).
The reason - Synology is a *Nix based peripheral so you will be dealing with being the “admin” of an appliance that has it’s own GUI/UI, and it’s own set of security issues, app updates, etc.
The “soft cost” of managing a different platform can be more than what you bargained for and the slightly higher cost of adding storage a different way or using another computer as a “virtual machine server” while using all the same management skills might be the better deal.
Also, macOS still has software-based RAID support for two drive RAID mirror built-in so you can get two relatively inexpensive but fast USB-C or USB 3.1 drives and make your own storage system relatively inexpensively now.