No SIM wouldn’t really make any difference, all (US) phones must be able to make a 911 call even if they aren’t currently in service (that’s what SOS in the signal meter means too, you aren’t roaming or don’t have roaming turned on but the rival carrier’s tower that you are hitting legally must allow you to make a 911 call). Just because your carrier doesn’t support those frequencies anymore doesn’t mean there aren’t other carriers that do or that the mobile cell tower spoofers won’t still scan for those frequencies. If you trust that it will stay in airplane mode and that the custom rom’s Bluetooth stack is locked down enough to not ping any Find My devices or similar then maybe but standalone Meshtastic device sounds like the best bet to me, or maybe something like IP over HAM radio. Maybe it’s time to dust off the Cybiko!!!

An anti-static bag wouldn’t do anything. I assume you mean the Faraday cage bags, which would prevent your phone from pinging anything as would a hardware switch but wouldn’t prevent them from searching it if you get arrested.