I used to use a DSLR for photographing minis, but it was a lot of effort to setup. Which meant I didn’t take that many photos. I have been using iPhones for several years to take photos of minis, and they work great. I have used a few different models over time, 3S, 6 and XR, the newer one has always been better than the old one.
You certainly don’t need the newest iPhone just to take photos of figures, any of the more recent ones will do.
More light always helps - I often use a piece of white card underneath the camera to bounce a bit of light back onto the figures. I have a cheap small light box which I sometimes use, and this definitely helps. I have found phone cameras struggle with white backgrounds - tending to under-expose significantly, using a dark or mid-tone background definitely helps the camera get the light settings right.