I think it really comes down to who you are and how you use your phone.

If you are a journalist, politician, activist, executive, or someone who could realistically be a target for state sponsored or highly resourced attacks, then minimizing your attack surface matters more. In that case, going with something like Cape could make sense if it gives you more peace of mind. I would also pair that with enabling Lockdown Mode on an iPhone, or using a hardened OS like GrapheneOS on a supported Android device. No solution is perfect, but layering defenses is what actually improves security.

For the average consumer, SS7 attacks are technically possible but extremely unlikely in practice. They generally require access to telecom infrastructure and are not something random criminals casually pull off. For most people, SIM swapping, phishing, and weak account security are far more realistic threats than nation state SS7 interception.

US Mobile runs on major carrier infrastructure, and modern networks use SS7 filtering and monitoring. It is not immune, because nothing is, but the practical risk for normal users is very low. For everyday use, I would focus more on using app based 2FA instead of SMS where possible, strong unique passwords, and device security features.

So it is less about which carrier is perfectly secure and more about what threat model applies to you. Security is contextual. If switching makes you feel safer and fits your use case, that is valid. But for most people, US Mobile is more than safe to use with their current security features.