I’m pretty sure I could go narrower at some point so I figured I must have changed some setting or something that stopped it… what should I change so it can go narrower again?
You didn't change a setting; you're mistaken. The physical limit for the aperture diaphragm is f/8. The reason for this is because there is no advantage to stopping down farther.
The pixel pitch of the P900 is 1.34um. At apertures smaller than f/2.8 diffraction is already causing reduced contrast/sharpness. At apertures of f/4 and smaller the sensor is losing resolution due to diffraction. And by f/5.6 the loss of image/sensor resolution will be apparent in an output image. At f/8 the output resolution due to diffraction is only about 2MP, which is only minimally acceptable for smaller display sizes and non-critical work (primary green centric wavelengths).
If you could stop down to f/11 the output resolution would only be about 1 MP (of actual detail w/in the depth of focus).
It sounds to me like you are setting up the camera as typically suggested in an online guide... But those guides assume you are using a DSLR with APS (f/8) or FF (f/11) sensor sizes, and they completely ignore effective focal length and subject distance (although they may have assumed near infinity?).
Because your camera has such a small sensor you will be using a much shorter focal length compared to a FF DSLR; a comparable aperture to f/11 on FF for DoF would be around f/4 on your camera. Whatever FL/zoom you choose, I would suggest leaving the aperture at maximum available (lowest #), I would also suggest using a tripod and setting the ISO to 100 with the SS at max/15 sec (ISO 200 and 8 sec would be my second choice). Then use a hat or similar to block/expose the lens in order to record the fireworks during the long shutter time. The duration of the firework explosion is really the exposure time. And using this method you can stack multiple fireworks in a single frame (by covering/uncovering the lens repeatedly).