Forget about syncing Veam backup files in the cloud with Synology cloud features. Unless your data is very small and your upload speed is huge, it won't work good.
The Synology NAS devices, are really good devices with a very nice and easy interface, but when you put your backup files on there (using iSCSI or CIFS) you will only use it as a means of storage and you need to appoint a server (or the B&R server itself) as the Data Mover.
A server (not antique) with a decent RAID controller and a bunch of disks in RAID 5 will out perform a Synology RS816 for sure. You just have to configure the RAID, partition for OS and backup storage and install Windows. That's it. In Veeam B&R console you just add the created server as a backup repository.
So my 2 cents, Synology devices are great for storing your backups, but an old(er) server (not antique) will have more performance, and you can use that server as Data Mover and perhaps as Proxy in Network backup mode.