Apple Mail and Apple Calendar are fine to replace Exchange, as is Thunderbird(see 1), but Mail is more turnkey (1-click pairs with MS Exchange)

You can downgrade your O365 licenses to Exchange Plan 1 and keep your email hosting at a tiny fraction of the price of full 365 suite.

(1): Beware thunderbird has an open and unsolved bug that randomly deletes all your emails, kind of like a 1d1 million dice roll.