The gimbal is still the most practical option for smooth video with your phone, that's true. The DJI Osmo Mobile is pretty solid, but what people don't usually mention is that the iPhone 15 Pro is quite heavy and some older Osmo models might struggle to keep perfect balance for long periods. If you go with that one, make sure you grab the most recent version that can handle your Pro's weight properly, or you'll end up draining the gimbal's battery in twenty minutes because the motors will be working at maximum.
That said, you've got other options worth exploring. The Zhiyun Smooth 5S is an interesting competitor, more affordable than some DJI models and it works like a charm. There's also the DJI OM6, which is more compact if what you want is something that fits in any backpack without taking up too much space. The real difference between brands is more about the software (tracking modes, video stabilization) than the gimbal's actual mechanics.
What matters most is that before buying anything, you check that the brackets fit your Pro properly without leaving your phone hanging. I've seen people with expensive gimbals who end up with their phone tilted because the mount doesn't close right. That ruins the whole shot no matter how good the gimbal is. Watch videos of people using exactly your phone model with the gimbal you're considering, not just the manufacturer's website.