This is where I would start. Presumably you’re after more than simply ‘calendars’ and the Nextcloud Stacy integrates with ( sometimes built-in, depending on dep

This is where I would start. Presumably you’re after more than simply ‘calendars’ and the Nextcloud Stacy integrates with ( sometimes built-in, depending on deployment ) caldav services. It’s also arguably a contender to replacing the M365 stack, with webmail, notes, chat, collaboration even including video