Posting here instead of the RCS megathread since this is not about getting RCS to work, but rather discussion about Google Messages in general. That thread should probably stick to on-topic technical support for getting RCS to work to prevent that thread from becoming more bloated than it already is.
As an American where the significant majority of people stick to their default messenger, this affects me more than most. But, I still don't think Google Messages/RCS is worth it anymore.
Plainly stated, I now believe it is privacy theater, especially in the current iteration of Android. Google scans messages on Google Messages, plain and simple. On stock at least, their Gemini AI is now part of it and it's very likely they are sending your message data to their servers in some shape or form. GrapheneOS likely shields some of it, but potentially not enough. It's unclear since Google Play Services and Google Messages are closed-source. Yes, it's sandboxed, but RCS requires you to poke many holes in the sandbox through ample permissions, always running play services access, device identifiers, etc. just to work. You also can't even compartmentalize it to a separate profile, it has to be on the Owner.
For an unrelated reason, I had to temporarily revert to stock PixelOS for the first time since this AI stuff took over and it was an eye opener of how intrusive and surveillance heavy Google's Android is. Even after 12+ cumulative hours of playing with settings, I was still stumbling across toggles and menus somewhere that gives Google permission to view and analyze sensitivite data. The system intelligence and AI has completely infected the OS and Google apps. Using the phone felt as private as a 1984 telescreen.
Recently, news came out that Google is directly supporting the ability for Google Messages conversations to be directly shared to employers on work-managed phones. Client side scanning makes promises of end-to-end encryption privacy theater. It's an effective backdoor and just like other backdoors, there's always the possibility it can be used on others as well. This is Google afterall. This shouldn't affect GrapheneOS phones, at least not yet, but this may already affect Android users you're messaging using RCS unless you verify with each person if they're using a work managed phone.
Even without directly sharing messages, Gemini will still be scanning people's Google Messages on newer Android phones since hardly anyone cares enough to disable it (if it's even actually possible). You should assume absolutely zero privacy protection for SMS/MMS and RCS messages going forward, regardless of e2ee promises.
I was in denial about this because I wanted a GrapheneOS phone without convenience tradeoffs, but I must adapt to changing environments and technology. I switched to Linux this year because of Windows Recall and Copilot and this is very similar. My threat model didn't change, my view on the technology I am using did.
As for messaging iOS users, there are still no privacy benefits, not even promised ones, since end-to-end encryption is still missing. The only thing you're doing is making conversations a little less frustrating for iOS users.
I'm going to take a play from the iOS user playbook and pull an uno reverse on them. When the messages start having issues, like group chats break, photos come through blurry, etc, be the first to complain. Go on the offensive. Because this is what iOS users do and are quick to tell you to buy an iPhone to fix the problems they're experiencing. Similar to most social dynamics, the first to complain wins. Ask them to switch to Signal before they tell you to buy an iPhone.
Ultimately, the only thing RCS accomplished was create a half-functioning compatibility layer to placate iMessage users' obnoxious demands and serve as an excuse to not switch to an actual privacy respecting messenger. iPhone user entitlement breeded this dynamic where Android users assume they are second class. As a GrapheneOS user, you shouldn't feel second class when you're using arguably the most private and most secure mobile OS available.
If people refuse to switch to Signal, so be it, you're not losing much of anything privacy wise. Or you know, do what iOS users do...keep complaining, little by little. Put it on them to install Signal to fix the messaging issues YOU'RE experiencing.
The one thing I've picked up on about tech normies, especially IOS users, is that they don't even know what RCS is and they don't care enough to learn. They still see green bubble vs blue bubble, except now, green bubble is sometimes less annoying, but also sometimes doesn't work at all. (Truly, even RCS on stock is unstable and unreliable, is carrier specific, doesn't work in some countries, and is device dependent. Seriously, it just sucks. Signal does everything better even outside of the privacy protections).
The most immediate perk in removing Google Messages is that I was able to dramatically rein in the play services permissions. I actually completely uninstalled it from the Owner profile and now update my play store apps via private space that I open once a day. Outside of privacy benefits, I've seen a big jump in battery life, even with Signal running in the background and Thunderbird pulling updates every 15 minutes. I may look into replacing Signal with Molly to supposedly save even more battery life.
I also feel a lot better running Pixel Camera and Google Photos with no network access. Before, there was always the lingering thought that they could bypass this restriction via IPC by going through play services, which did have network access.
For my remaining Google apps that I'm still in the process of removing, they're now neatly tucked away in my private space, so I've lost little usability. But this is my biggest leap in my degoogling/privacy journey since switching to GrapheneOS and I can only do this because I ditched Google Messages.
To replace Google Messages, I switched to Fossify Messages for SMS/MMS. If there are better recommendations, let me know. (I need a reliable search feature and ideally a timeline feature to easily jump to older messages, neither of which the default Messaging app supports).