“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.”
Beeper has worked well for me but I think you might still have to get on a waiting list. I have my one Signal account on any device I pickup now.
https://www.beeper.com/
I recently found out about beeper. The guy who made pebble and sold to Fitbit is making this.