What's the fastest, safest, and most reliable way to transfer from Synology to TrueNAS?

[jgreco]

And I get why people would want an OpenVPN server, just as I get why people would want DDNS support--but in both cases, as a built-in service to the NAS doesn't make sense to me

To be properly implemented, it needs to be able to manipulate the host system, sysctl ipforwarding, routing tables, certificates, and firewall. It is NOT just a service daemon such as services like TFTP, WWW, FTP, rsync, WebDAV, et. al. which are just some random daemon executable with maybe some argv flags and perhaps a config file or two. If you can discard your preconceived notions about "value-add services" and look at it from an implementation point of view, there's a lot of stuff that is effectively trite and should even be field upgradeable and even hackable independently of the OS (did someone mention "nut"?). Services like tftpd are very easy to set up, even from source. It's stuff like OpenVPN that requires deeper integration.