“When you use Signal, your data is stored in encrypted form on your devices. The only information that is stored on the Signal servers for each account is the phone number you registered with, the date and time you joined the service, and the date you last logged on.”
Reactions are a work in progress and one major client (Conversations) does not have them yet, the others do. It has been federated since early 2000s and had multi user rooms with history for at least the last decade that i’ve been using it.
Try it out one day, even if you continue to use Matrix day to day.