If you really need the live migration and true HA, a single Synology unit is simply not enough. It would turn into a single point of failure in your case. Also, Synology is a great solution for storing backups or being a file server, but it's not really a good fit for the primary production storage.
There are a lot of software-defined solutions that can help you building a proper redundant storage for your cluster. For example, you can take a pair of hypervisor hosts, fill them with required amount of storage an use something from this list:
HPE VSA - Linux based HA storage solution. Requires 2 physical nodes and 1 witness.
StarWind Virtual SAN - Windows based HA storage solution. Requires 2 nodes.
Unity VSA - Linux based HA storage solution. Requires 3 nodes.
All of above solutions also have free versions available.
Instead of building HA cluster from scratch with KVM, DRBD, cLVM and Pacemaker (or whatever you prefer), I would recommend you taking a look at Citrix XenServer which is free and has most of the necessary features already pre-installed and ready to use.