Thanks @Sayali-MSFT for the answer, which is this:
In scenarios where you're not using bot frameworks and not dealing with real-time interactions (i.e., receiving messages and responding), obtaining the turnContext.Activity.* is not necessarily required. If your goal is purely to send messages at scheduled intervals, you can proceed with the REST API calls as you've indicated. Use the endpoint smba.trafficmanager.net/teams/v3/conversations to create or retrieve the chatId for users.