I don't know about secure, when SMS itself isn't by default, but aside from that I've personally been using QKSMS for a few months, after moving away from Google Messages and I found no issues, it's solid and quite featureful.

I know it's not a Signal fork with SMS support, which I'm sure still exists, but I wouldn't use it because it would have to keep up with upstream and maintain the SMS feature too, so it most likely will fall a bit behind, which isn't the best thing to let happen on an actual secure communication app

Matrix is encrypted and decentralised, and has bridges for SMS: https://matrix.org/ecosystem/bridges/sms/

I've never heard of anything that offers secure messaging like Signal, and insecure SMS.

But there are lots of SMS apps that can be used along with Signal. No need for them to be the same app.

I love Signal, but just a PSA, if you convert between Android and iOS, you're SOL on migrating your existing messages. You can migrate Android to Android or iOS to iOS, but not between the two platforms. There are at least applications that will let you migrate SMS between the two.

Your preinstalled SMS app is usually the best way to receive SMS, I wouldn't give and unnecessary app permissions to access them!

I think it's a bad idea to mix the two tbh.

Beeper uses Matrix as it's backend. Love it!

The App Molly is an alternative App for Signal that kept SMS Support i think

I'd just use a FOSS SMS app like Simple Messenger from F-Droid or if you have GrapheneOS or CalyxOS, use their default messengers.

I switched from Simple sms messenger to QKSMS. It's really nice.

https://www.simplemobiletools.com/

Silence but its unmaintained