Your instinct to not relax the exterior oblique seems to be the most likely answer. And, another part to consider, if a muscle is already engaged to its full power, engaging it more tends to be difficult. Similar to how we bend our knees to jump and land a jump, relaxing and contracting muscles can be a part of a dynamic skill. There is a curve but not a complete softening of the side.
Hip key roll ups are difficult. Dynamic ones can take years, slow ones may never happen for all bodies, they are so variable with all of our individual torso to leg lengths, what kind of range of motion we have, what our core strength is, how our quick twitch muscles system works, not to mention the timing. Please give yourself space, give yourself time, and give yourself a really strong driving bottom leg.
