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FetalNevada The stock SMS app is not an alternative. It's a joke. It doesn't even give you the option to hit "return" and enter breaks between paragraphs
The AOSP Messaging and keyboard apps are pretty basic. Fixing issues or improving them has not been a priority. The project recommends to avoid using SMS. The keyboard app will be replaced. GrapheneOS has limited drveloper resources and has to focus them carefully.
There are other keyboard apps that do have an active 'return' while using Messaging.
If Im not confused Google Messages also works across profiles. If that is the case then the cross profile issue is due to other SMS apps not supporting this functionality. Its an 'exotic' requirement.
FetalNevada It has a predictive text "feature"
That is also the keyboard not the SMS app.
If you need another SMS app to work for you, you will likely need to use it in the owner profile. If you want the isolation provided by different profiles, but still have immediate access to SMS, you could use Private Space and maybe even a work profile, rather than using secondary user profiles.