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
That's because SMS isn't end to end encrypted in general, you need a encryption client on both sides to achive E2E encryprion aka Singal to Signal or something comparable.
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.