Good that you got the Camera part working.
From my knowledge of policy-based VPN vs. route-based I have to disappoint you. policy-based (which is what dynamic VPN utilizes) is a point-to-point VPN connection and you cannot do hub-and-spoke topologies meaning that receiving traffic via one VPN and sending it out of another one is not supported.
This is at least how I understand the documentation.
Dynamic VPN cannot be done as route-based VPN so you're out of option - except having a seperate VPN gateway on side of the SRX, then it would work but possibly also be an overengineered and too expensive solution :-)
I hope my input was useful.