It matters even more if you need to maintain several-year-worth of information concerning hundreds of customers and share them with others. None of the apps you mentioned above offer the essential privacy and security you need for business use like that. A bit old and outdated, but still a case in point. And, for that, what makes you think Notion is good enough for storing hundreds of customer data? And, not to... Source: almost 3 years ago

I use NextCloud calendar across multiple devices but also with multiple calendars (1 for work, 1 for personal stuff, trips, events, etc). Some of them are personal and others are shared either as CalDav or as URL (web pages). I also linked some of those calendars to https://harmonizely.com which allow collaborators or total strangers to book a moment when my calendar is open. I can also import .ics files when... Source: almost 3 years ago