The alternatives already exist and they are build on Nostr and it has different apps/clients to interact with that protocol. For example Olas is a Instagram-like and tik-tok-like app, and there are also many twitter-like or mastodon-like apps such as Amethyst or Damus
I use Logseq in my PC and my phone and I unse Syncthing to sync the notes accross my devices.