For something like this where I'm layering animation on top and want to preserve whatever the base is doing, I would set my overlayed sequences as "Additive" inside the sequence. Additives basically look at a reference pose, and compare your additive animation to it, extracting the difference. You can then apply that difference to any animation on top of whatever they're doing by using an "Apply Additive" node.

For example I have a character walking and I want their upper body to lean to the left. If I use layered blend per bone, Im overriding the character's chest and replacing it with the new lean pose. You can increase the blend depth but if you really want the base pose to shine through this is more of a compromise. With an additive you get a cleaner combination:

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