First things first, Signal Protocol, the cryptography behind Signal (also used in WhatsApp and several other messengers) is open source and has been intensively reviewed by cryptographers. Finally, it’s trivial for Americans to setup a second Signal account using Twilio or Google Voice, so you can compartmentalize your furry posting from the phone number your coworkers or family is likely to know. From what I understand, the Signal development team has always been much more concerned about privacy for people that own mobile phones, but not computers, than they were concerned about the privacy of people that own computers, but not mobile phones.