Pondered your message while going over pics taken during a cycle ride and walk : those pics, with a still camera, show excellent detail in the darker shadow tones, but for example white streetname signs can be washed out, as are the clouds : exactly the opposite of your action cam.
Looking at it from a design engineer’s point of view, would be guided by different aspects for a ‘Digital Still Camera’ (the one which will be used intermittently for short exposures), and for an action camera which may be on for longer periods -even hours at a time- and aimed at rather brightly lit scenes - surf, snow, (reflections of) the sun.
When the sun shines on the lens, and thus on the sensor, you’d have the same effect as with a ‘brandglas’ (Dutch word for a lens which focusses the sun’s rays on a minute area in order to heat that spot with the effect that -if its combustible- it’ll eventually catch fire).
Another effect is that allowing a relatively large amount of energy (light is energy, think of solar panels) on to a small sensor for an extended period of time will heat the thing up, and accellerate chemical decay of the light sensitive elements.
From the design point of view would minimise the amount of light reaching the sensor, thus take the brightest area as the basis for the exposure : that’d be the sky, which in all your pics seems well-exposed.
How to work around this? Find that aiming the camera somewhat down - say with the horizon at 1/3 from the upper edge - can be sufficient to get the camera to lighten up, as it were.
You might also search for what the Garmin manual refers to as EV-bias (and perhaps EV-lock) : please do try out and experiment, as Garmin’s manuals are at times short on relevant detail.
Hope this helps, looking forward to your report on the results,