Relationship between pressure, temperature and density in meterology
'Locally' and 'synoptically' are conflicting adjectives. Synoptic means overall, or large horizontal scale- think thousands of kilometers.

You are right on some accounts. Synoptically, pressure doesn't change much, depending on your qualitative definition of 'much.' On a percent scale or day-to-day experience, absolute temperature fluctuates more than pressure. But the gap of those fluctuations result in changes of density, leading to changes in buoyancy.

The vertical momentum equation :$$\frac{\partial w}{\partial t}+(\vec{v}\cdot\nabla_H)w+w\frac{\partial w}{\partial z}=-\frac{1}{\rho}\frac{\partial P}{\partial z}- g + \nabla^2w$$ when applied to synoptic scales can be treated as approximately hydrostatic: $$\frac{\partial P}{\partial z}=-\rho g$$.
From this equation, as well as the ideal gas law, the hypsometric equation may be derived.

Although the vertical gradient of pressure is large, gravity is also large, which explains why the atmosphere is not lost to space and vertical motions aren't usually large in horizontal scale.

Using said hypsometric equation, it can be shown that changes in vertically-averaged temperature lead to synoptic scale changes in surface pressure.