If you're on Android, as well, look into BlueBubbles. It bridges iMessage from a MacOS system to most any device/OS. I've used it for years now with my partner's family with very little issue (all of which were resolved with a restart).

Hard part is getting a MacOS system. I started with a VM, but eventually landed on a ~$100 Mac Mini 2014. Both solutions worked well, but the former is against Apple's TOS and requires spoofing things, so it's ultimately much less reliable than actual hardware.

I don't know much about the creator of GrapheneOS. What's the bad about them? I know they're a little dogmatic, as security/FOSS folk can sometimes be, but I've not heard anything beyond that.

If you are either A) bootloader unlocked or B) using a custom ROM via an exploit, your system is freely open to modification by a physical attacker, regardless.

And if the prospective protestors are comfortable, they'll stop short of their goals. Perhaps very short, if they start at all.