Let’s say the US gov starts agressively prosecuting dissidents and certain minorities (they already do): can and should we still use signal in this case?

Definitely no. Russian government already is aggressively prosecuting dissidents and you can't join Signal there. I don't know whether it's due to sanctions or if the government is blocking 2FA SMS messages. In either case, it is impossible to join without a phone number confirmation. At least I wasn't able to. I don't see the USA being that far off with all the recent TikTok drama.