I built melonpost a long time ago mainly for my own family to send each other physical postcards from an whatsapp chatbot. Couple of month back i replaced my 10 year old clunky chatbot with one of the leading LLM models and the process just became effortless (vs only usable by power users AKA my mom).

Turns out it's lot's of fun too when using it from a coding agent sending colleagues snippets of their weird code comments. Linking it to your favorite image generation etc.

All the best of luck with Shoutium! I still love the concept of a local messaging app and it was great fun in active areas. But like you said getting the critical mass required is always tough.