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.
I had never heard of BangleJS before and wow, that thing is awesome!
I'm neither a JS or Android/iOS developer, and this thing makes me want to try making a Fitbit-like fitness tracker and companion app.
I might seriously get one. I've been on a privacy/declouding kick lately and have somewhat missed my smart watch. This thing looks like great fun.