Hello,
Yes, That is right if you turn the SRX 3600 in Packet mode it will turn into a router and will lose all of its protection features.
Unfortunately on High End SRX devices (like your SRX 3600) it is not possible to have only selective traffic in packet mode.
Thanks,
Pulkit Bhandari
Please mark my response as Solution Accepted if it Helps, Kudos are Appreciated too. 