Is it possible to configure 1G/Full on SSR WAN interface?
No.
Per IEEE industry standard specs, auto-negotiation is mandatory on 1G and 10G RJ45.
Reference:
SSR documentation, link speed options
https://www.juniper.net/documentation/us/en/software/session-smart-router/docs/config_command_guide#configure-authority-router-node-device-interface-link-settings
Juniper EX KB on the topic:
KB22315 [juniper.net] https://supportportal.juniper.net/s/article/EX-The-behavior-of-Gigabit-Ethernet-interfaces-when-no-auto-negotiation-with-1000-full-is-configured
Wikipedia:
https://en.wikipedia.org/wiki/Gigabit_Ethernet (includes other references)
"Autonegotiation is a requirement for 1000BASE-T implementations"