Yeah, ChatGPT voice is great experience vs. Siri on that phone. In case you haven't done something like this already:
1. In Shortcuts app, make shortcut named "AI Voice Mode" (or whatever you want, YMMV)
2. Set it to run the ChatGPT action "Voice Mode" (requires at least the minimum paid tier, I think)
3. To trigger, say "Hey Siri, AI Voice Mode" (or whatever you called the shortcut)
This is a pretty slick integration, but yeah, if it were baked in that would be all the better.