For most: yes, there is a risk that the vendor has included a backdoor. There is also the risk that they are straight-up lying about how their service operates.
For Signal in particular: You can verify that their claims are true because you can audit the source code.
The Signal client is open-source, so any interested parties can verify that it is A) not sending the user's private keys to any server, and B) not transmitting any messages that are not encrypted with those keys.
Even if you choose to obtain Signal from the Google Play Store (which comes with its own set of problems), you can verify its integrity because Signal uses reproducible builds. That means it is possible for you to download the public source code, compile it yourself, and verify that the published binary is identical. See: https://github.com/signalapp/Signal-Android/tree/main/reproducible-builds
You might not have the skills or patience to do that yourself, but Signal has undergone professional audits if anyone ever discovers a backdoor, it will be major news.
You are more likely to be compromised at the OS level (e.g. screen recorders, key loggers, Microsoft Recall, etc.) than from Signal itself.
Can't go wrong with publishing order. I refer to the Coppermind wiki for that: https://coppermind.net/wiki/Cosmere#Bibliography_of_the_Cosmere
I personally started with Stormlight Archive, which I wouldn't call optimal, but hey, I have no regrets. In general I'm a "gates open" kind of fan, so I encourage readers to go in whatever order they like and enjoy any extra mystery that might come along with it. :)
The only ones I'd really recommend against reading without context are The Sunlit Man, and to a lesser extent Yumi and the Nightmare Painter. It kind of pains me to say that about Yumi because it's such a fantastic book and I'd hate to discourage anyone from reading it. But you'd need to comfortable rolling with confusion if you've never read any Stormlight Archive, because
the narration is written in the voice of Wit and it has many references to Roshar
That said, Lost Metal includes connections to
Elantris, and The Emperor's Soul
I wouldn't say that's required reading before Lost Metal. I personally read Lost Metal first and, again, no regrets. But now I do kind of want to go back and read a few chapters again given the additional context I have now.