Two important points you have to keep in mind:
- The dynamic range (DR) of PO film is incredibly narrow. It is even more limited than slide film
- The OneStep 2 allows for barely any precise exposure compensation, unless when shooting in manual mode.
The first point is likely the reason your photos are oftentimes partially under- or overexposed. In case you don't know, the DR of a film indicates how 'wide' the range of light is the film can capture without losing detail. In other words, it indicates how big the difference between light and dark areas in a scene can be before the film starts losing detail in these areas.
For conventional film, and most digital sensors, the DR is quite high. A film with a DR of 12 stops will have no problem with photos of a sunny street with dark shadows.
PO film, on the other hand, has a whoppingly low DR of 3 stops. That means it over- and underexposes very quickly, and there is little to none you can do about that. Shooting on a cloudy day or in other 'flat' lighting conditions is all that will help you.
I never use the flash, as I don't like how it makes the photos look, but I can give you some advice on using the L/D slide. The SLR cameras originally made by Polaroid had a L/D wheel, which you could set to quite precise settings. The new OneStep 2, unfortunately, has a slide, which only allows for three settings: neutral, lighten, and darken.
When shooting with a folding type Polaroid, you may want to set the L/D one notch to lighten, which is a minimal change you're not able to perform on the OneStep 2. If I were you, I would leave this slide centred and instead use the manual mode on the PO app. Although far from ideal, this allows you to adjust exposure accordingly, and precisely.
Determine the area of interest, and adjust exposure on basis of the lighting on this area! You could do this by pointing the camera at the area of interest and letting the camera do the light metering for you (before recomposing), or by guesstimating. Both methods require some knowledge on (manual) metering.