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