Bulk messaging to 1:1 chats

marbulk I would suggest not thinking about these as 1:1 scenarios, because they clearly aren't. Use Teams!

In your examples

1. there is a summer trip and a message about a sensitive personal topic has to go through a private message to the people that have signed in for the trip.

Create a Team for people going on the trip, HR then post a message an @mention the Team so everyone gets notified. They can add a Microsoft Form to their message to securely get specific answers back which are automatically compiled into a table in excel.

2: a system administrator collects information how employees manage company's private data locally.

Use a Company Wide Team (if less than 10K users) and a similar approach to the above.

When posting a new conversation, the author can press the little format button, then on the expanded compose choose if they don't want others to reply on the thread, as it wouldn't be appropriate. Collecting data via Microsoft Forms is a far better experience for people submitting data, and enormously more convenient for the person collecting it.