I just lean the camera on a rock or something and use the 10s timer. The stillness it has during those 10s enables astro mode.

I’ve never hit a low light level requirement, but I only use the feature at night. Even when there’s a “bright” object in the frame, like a house with it’s lights on, it takes a 4 minute exposure and it comes out great.

OpenCamera has manual controls but can’t change lenses on a Pixel 7 Pro. Obviously, it doesn’t do intelligent image stacking, but it can do long exposures.