Partial ranges of motion can improve activation and recruitment of the core muscles. And it can re-educate your core so that its strength can be transferred back into the compound lifts.
Here's what to do: Use a pad to elevate into a more passively flexed position at the thoracic spine and rib cage. This position will improve the strength curve of the rectus abdominis, allowing a more targeted way to overload these tissues with load or metabolic stress without jeopardizing the spinal position.
If you struggle to engage the posterior chain to stabilize the pelvis, a great variation for the partial range of motion crunch is placing your legs on a weight bench in a 90-90 position.
From this position, add a pad to the back to create the perfect position to gain muscle tension and recruitment, and to safely train the spine using a controllable and strategic flexion. It's best to get the hips flexed to over 45 degrees which helps the pelvis to stabilize more effectively.