To avoid problems of Kesiva, they have designed a preset keyboard with 120 Keywords plus 30 words you can customize with which you don’t write but simply select the words to become part of your sentence. Any everything is delayed reaction, and user history is deleted periodically, so as not to be permanent.
(Besides for all the other issues,) Why isn’t there a separate problem with the word showing up on the other person’s screen after you hit “send”? Unless I am mising my mark, by creating a P’sik Raisha (even if you claim it is indirect) that writing will appear elsewhere, that should be at least a D’rabbanan (even if the Kesiva is not shel kayama).
A real “Shabbos app” might work exclusively with voice, so that no words would be created. Then I could see the question being asked (since it is well established all over shas that dibbur is not a maase).