CalDav Server:

• Baïkal CalDav Server (needs only PHP, SQLite)

Clients:

• PC: Vivaldi, Thunderbird (and KOrganizer for importing ICS files and syncing them to CalDav Server)
• Thunderbird also for contact management
• Android: DAVx⁵ for Syncing Calendars to device, ICSx⁵ for syncing external WebICS Calendars.
• org.fossify.calendar as GUI to the calendars.

Calender Sharing:

• Sharing Calendars, log in to <YOUR SERVER>/dav.php, navigate to calender, at the bottom share with other user on your server (you may need to create those users), see https://devblog.yvn.no/posts/baikal-caldav-hosting/