If you want the router acting as DHCP Server and the SRX to act as a switch with security features, then you are looking to configure the SRX in transparent mode.
Please share:
> show ethernet-switching global-information
> show interfaces terse
> show interfaces extensive
> show security polices
I also found that application "any" is not supported on the security-policies, so you might want to change that:
A security policy for transparent mode is similar to a policy configured for Layer 3 zones, with the following exceptions:
Ref: https://www.juniper.net/documentation/en_US/junos/topics/concept/security-policy-transport-mode-understanding.html
Based on the same document it looks like the DHCP packets should be allowed:
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By default, Layer 2 forwarding performs the following actions:
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Make sure you dont have the option "block-non-ip-all"configured else DHCP traffic will be dropped.