FWIW, some tips:
Onion skin always looks transparent here. For example...
In this case, my onion skin frames are spaced on twos, but that's because I'm drawing the FBF on twos.
I'm also using the Selected Layer Only option, which can filter out everything else in the project from the onion skin. In the above example, I have only the FBF layer selected.
Additionally, I have Colored Fills disabled. This is option is useful for identifying before and after frames, but it can also darken the image. I don't usually find it necessary when drawing FBF, so I disable it. When I do use Colored FIlls, I use the desaturated colors seen in the image. (I'm not sure if these colors are default. They could be.)
Occasionally, I like using Outlines Only or Draw Behind modes. This is completely situational, but they can be very helpful in the right situations.
I almost never use Onion Skin when animating a rigged puppet, at least not when the character isn't moving around the environment very much (like 'the Artist' character in our tutorial videos.) I might enable it when the character is extra active, like when fighting, performing acrobatic motions, or dancing, but most of the time, it's unnecessary for puppet animations. For example, in
Space Tommy: Mission Critical, I probably used Onion Skin only in the scene where he captures the golden asteroid. I did use Onion Skin for the two explosions, but those were Moho FBF, not puppets.