Nextcloud is both an enterprise solution and a free self-hostable solution. I self host my own nextcloud instance on an old mac mini running ubuntu server in a docker container.
To always stay connected I have wireguard vpn running on al my mobile devices so they can always reach that server (the iOS apps have “on demand connection” enabled on unknown networks, including 4/5g)
This hosts all my reminders, calendars, contacts, files and photo’s and since moving to this self hosted solution i’ve not looked back. Tbh: it hosts them for my entire family, and they don’t even know we’ve switched from iCloud.
If you want to try it out you can always just look at the nextcloud website and find one of their offerings, the partners do have free trials available.