So this is work in the pipeline, and there's a bunch more polishing that's needed, but I will definitely be writing more about it soon, and I hope that will ans

So this is work in the pipeline, and there's a bunch more polishing that's needed, but I will definitely be writing more about it soon, and I hope that will answer your questions. But I'll give a brief teaser here anyway. The new spline is a hybrid between Spiro and cubic Béziers, with (I hope) the best features of both. Spiro is good for smooth curves but not so good when you need to crank up the tension. Béziers, by contrast, can do tension just fine, but aren't very smooth (one technical manifestation is that curvature maxima tend to happen near the segment endpoints, but rarely on them, especially for S-shaped curves). Something that's particularly difficult for Béziers is a smooth transition from a straight line segment into a curve, which is important for U-shaped letters. Unless you take special care, the curvature jumps right up to some nonzero value at the join point. But with the new spline, it's continuous by default, unless you explicitly drag the control handles to make it not so. It's difficult for people to learn to draw well with cubic Béziers, but obviously people climb that learning curve (so to speak) and get great results. Even so, I wonder if there's a bit of Stockholm syndrome, of being attached to that way of working just because of the amount of pain that's gone into the process of learning it.