OP, if by "old iPhone XR" you are really asking if you can use a cellular mini to stand in for an aging iPhone, the answer is yes. Try a good VOIP app using a temporary number that generally comes with them until you find one you like. If it is good enough, port your main number and your Mini will basically become a bigger-screen phone. Calls in and out, texts in and out, Mini will ring, etc. All works fine as a phone. Texts to and from other Apple Messages users work fine. Texts to and from non-Apple people work within the VOIP app.
Other than a few very brief periods of time, I've rarely used iPhones, preferring iPad mini + buds with mic. It all works... much like how iPad mini can also be a flash light, a camera, a controller, an iPod, etc. Telephony is just an another app. iPad cellular is MUCH cheaper than iPhone cellular plans. All other apps generally work better with more screen RE.
The most notable "catch" for you might be in whatever dependencies Watch has on owning an iPhone. I favor traditional watches myself. But it's easy enough to experiment with the concept (free VOIP app + free test number) and see for yourself.
