Need help verifying Signal Android APK

I’m downloading Signal from the website, even tho they don’t seem to want you to, because I’d like to be able to completely rid myself of the Google Play Store (as used with Aurora which has its own problems from time to time), and I believe that this version auto-updates or at least tells you when there is one. Following the instructions here using the apksigner in the repository just gives me lots of error messages. I’m using Linux Mint 21.1 (and just because I’m using Linux doesn’t mean that I know what I’m doing). I think I read somewhere that the apksigner in the repos is (of course) broken and I may need a newer version but I don’t know where to get it. Any help with this would be greatly appreciated.