As an old school Linux desktop person from back in the day: whatever happened to Evolution? The whole reason it was created was to sell an Exchange client plugin.
I use Evolution every day with the evolution-ews plugin for accessing an Exchange server. It's still a bit clunky as it always was, and has some issues when switching between wired/wireless networks, but otherwise works fine for email, calendar, and contacts.
Same, once or twice a week I have to close evolution and open it again to get it to resync but otherwise it's less painful than Outlook 16 on W10.
The original plugin worked with the WebDAV support in Exchange, which was removed some versions ago. To make it work properly you need to support the MAPI protocol Outlook uses, which is undocumented and (at least used to be) obfuscated.
evolution (and evolution-ews, which is open source) work very well for office365. I personally only use the calendaring as read-only, but I assume it can create events. Email works fine.