What about having an efficient communication between users, you could use Dynamics 365 InApp Notification to interact with user without disturbing them and keep a nice overviews of your tasks.
You would like to migrate your notes to posts and vice versa in D365. This article shows one option to acheive it by using XrmToolBox. Read it.
Activities are entities in Dynamics 365 made to help an organisation understand the interactions it has with its customers and prospective customers. This article will present the main points related to Activities.