The only thing I can see is Linwood Flow but it looks like it’s a really, really long way off before becoming a primary calendar app.
If it could just store all calendar content in 1 local file, then that could be so easily auto-synced across devices with !syncthing@programming.dev. Does anyone have any leads?
You could use a free VM on Oracle cloud infrastructure and create regular backups in case thwy rugpull the VM (as it’s free)