Even though messages are chronological in order (time based), people might want to start a new thread on a previous message that isn’t the most current message. So the hierarchy isn’t just the time a message was sent, but it’s also based on the message that it’s being responded too as well.
As such, I’m not certain if using a time series database is the appropriate use for a chat app.
EDIT:
Question - What do you believe is the most appropriate data-store to use and why?
(Eg relational, nosql, document, time-series, etc)
But the feature you are mentioning like having threads could be easily be solved using like a document store ex; mongodb or any other no-sql db that could help you save time in both setting up things and querying.
Just my 10 cents.