The key in using off-the-shelf software, especially if you're not a capital-E Enterprise that can dictate new features, is to be willing to change your business processes so that they match the tools you're buying. Part of the purchasing process for SMBs should be (but almost never is), evaluating current processes, and which parts of those processes are arbitrary and/or can be changed, then finding COTS software that fits all the must-have/can't-change processes. Then you adjust the last few remaining things to fit the software.
But invariably you continue to follow the advice and buy a 2nd. And it won't integrate well with the first. By the time you get to the 10th system that doesn't want to talk to the others, you're employing dozens of people to do the data entry between them.
And these proprietary systems almost _never_ want to talk to the others. There is a reason Outlook doesn't talk to Google Calendar, and Google doesn't talk to outlook's calendar, yet somehow Thunderbird (open source) manages to talk to both. And I can't count the number of times Microsoft has managed to break their IMAP interfaces.
I've seen this dark pattern in so many large businesses now, I've come to believe it's the norm.
My O365 displays my Google calendars just fine.
My iOS and macOS calendar apps display both just fine.
I use a handful of Google calendars, but almost never use the Google calendar UI directly and interact almost exclusively via the Apple or Microsoft UIs to the Google store of data. (As in I might use the Google UI directly perhaps once a year, and that's generally only to get the access/path to the iCal interoperability URL.)
