Can a BMV-712 smart monitor be added to a MPPT 150/35 system to allow a Bluetooth connection, or is the dongle the only option?
If you mean a BlueSolar MPPT 150/35, then the only way to add BT to that controller is with a VE.Direct Bluetooth Dongle, yes.
If you have a SmartSolar MPPT 150/35 and you’re asking about VE.Smart Networking it with a BMV-712, then that is possible without additional dongles, see the VE.Smart Networking manual here: https://www.victronenergy.com/upload/documents/VE.Smart_Networking/20723-VE_Smart_Networking-pdf-en.pdf