Boiler heating gauge drops to negative pressure (psi) on cooling

Water cannot compress, only gasses like air and steam can. So if there is no steam an no air in the system, the expansion of the hot water as it pushes against the cooler pipes can be problematic. So most systems have an "expansion tank" that holds an amount of trapped air to allow for that change in pressure so that the pipes don't burst. Once everything is at the same temperature, no pressure shows. But the hot water should be flowing in the pipes, that's why you get the heat. When it turns off, the water cools faster than the pipes, so you get that negative pressure again for a while. In other words that part all sounds normal.

Heat given off into the room is based on the flow of hot water. So there is usually a circulating pump in the hot water piping system, often right next to the boiler. Do you hear that running? If not, it should be and that's your problem. Hot water could still be circulating a little bit due to natural convection, but you need the pump to make it work well. Even if it is running the pump may be bad, especially if it is old. The impellers are usually made of bronze and can corrode if your water chemistry is bad.