I have the DS918+ and it's great but I can't upgrade the network
would the Synology 12 Bay DiskStation DS3622xs+ be a better bet for this?
Yes, you could use USB 5GBE adapter, but for 6K I would rather go for 10GbE LAN. DS923+ and up to DS3622xs+ is the way to go (something with PCIE or 10GBE built in).