Try if this helps
route 192.168.2.0/24 {
next-hop st0.2;
qualified-next-hop st0.3 {
preference 6;
}
}Whats on the other side of the tunnel? If also SRX there is an option to configure BFD for static routes but I'm not sure if it works with unnumbered interfaces.
https://www.juniper.net/documentation/en_US/junos/topics/example/policy-static-routes-bfd.html
Regards, Wojtek