[Syonyk]

KaiOS supports WhatsApp. And, at least some of them support group texting somewhat competently, though not the older ones.

But the reality is that I'm fine with a lot of my communication being computer-based now (with a keyboard). I still manage some group texts, but tend to only actually contribute if it's something critical ("Can you make this date work?") and skip a lot of the random BSing in the threads.

If your requirements are "I want everything I can do on a smartphone, but without a smartphone," you end up in an impossibility, so just find the least offensive black mirror you can and go on with it. But if you're willing to sit back and figure out what actually matters from a phone, and what's a nice-to-have, there are plenty of other options out there.