You’ve pretty much just described ActivityPub and the Fediverse.

Anyone can spin up their own instance. You can self host on a machine in your house, or with any cloud provider. You can broadcast messages in Twitter-style or Reddit-style format. Anyone can navigate to your web address and see your messages. Anyone who federates with you can see it on their website. FOSS Android apps are available.

You can’t force anyone to actually read your messages of course, but that’s a different matter.