I noticed that nobody had really replied yet, so let me have a crack at it.
This configuration should work well enough, but especially for the remote access to the files, but even for the local acces, you’re also very reliant on the local network and your internet connection.
What I mean is that if your NAS has a 10GbE nic, but your local network has 100MB cabling/switches/router gear, you’re not going to have much use of the 10 GbE nic. So it’s difficult to answer that question at this time.
HyperBackup will allow you to do backups e.g. daily. Always consider doing a backup to an offsite machine if at all possible in addition to e.g. a local external HDD. Considering the amount of storage you’re aiming for I’d guess getting external backup in the cloud could be quite expensive. And if that’s not an issue, why not host the data there instead of on the NAS and then make a backup to your NAS? In that way you won’t have to worry about your internet connection as much. But I guess that’s not what you want to do.
Consider using Synology Drive to synchronize the files between the users and the NAS, this allows for the users to work with the files locally and then synchronize them again.
Security is always important so do make sure all users use https connections to the NAS or use VPN connections to the NAS.
Not sure where you’re located, so can’t really recommend anything, but a lot of what you’re mentioning you can probably just order online and then put together yourself.