Just a quick comment about Nextcloud. I host that on my media center (which runs Ubuntu) which is inside my LAN. It can sync CalDAV and CardDAV stuff. Just mentioning as a real world example. We also use it to share files, tasks, sync Joplin, etc. Really handy. Not sure I’d want to keep up with patches and security on the public net, but on my LAN I think it is fine.
Other more expensive direction, I wonder if there is a Synology product that would do it. People seam to rave about Synology NAS.