Damn they're making todo lists a subscription service now??
To answer the question: anything that provides a CALDAV backend (e.g. Nextcloud, Etesync, Radicale). Some are free with limited storage, but some are subscription based, but you get calendar, storage, other stuff too. You can additionally self-host a CALDAV server or Nextcloud to use these services gratuit. For a more minimal implentation, try plain text, markdown, orgmode, etc., and use Syncthing to sync between devices.